A Review on Phytoremediation of Decommissioned Mines and Quarries in Ontario: A Sustainable Approach

Abstract

Abandoned pits and quarries in Ontario, Canada, are on the rise due to industrialization, leading to ecosystem disruption and soil contamination with pollutants such as cadmium, cobalt, nickel, and barium, which may leach into nearby water systems. Current rehabilitation processes are slow to initiate, and therefore, the site remains in a contaminated condition for years. Phytoremediation, which involves using plants to remove contaminants from soils, is receiving increased attention for cleaning up decommissioned mines. This type of rehabilitation is normally practiced in situ by hand-planted and managed vegetation chosen for the specific purpose of contaminant removal. This study investigated the phytoremediation potential of indigenous plants as local seed sources to rehabilitate decommissioned quarries in Ontario. This study also investigated the potential of native plants to naturalize in the disturbed areas, thus providing a natural clean-up of the contaminants. Thus, if successful, this process will also initiate the re-establishment of native wildlife in the area. Through a literature review, 74 plant species were identified as capable of remediating 20 contaminants often found on the decommissioned quarry sites. The results may help ecosystem managers to adopt environmentally sustainable strategies to clean up contaminated sites such as decommissioned mines and quarry areas.

1. Introduction

The population of Ontario, Canada, is growing, and with it comes the expansion of cities and suburban growth. New infrastructure is required to meet the needs of new communities. The inclusion of roads, housing, commercial buildings, and industry means there is an increased need for aggregates and other materials from quarries and mines [1]. As a result, the number of abandoned pits and quarries across Ontario is increasing. The process of aggregate extraction leads to the deposition of contaminants, including heavy metals, hydrocarbons, and explosive compounds, in the soil, which results in environmental issues for ecosystems, particularly affecting water resources. Ontario has more than 6000 active mines and quarries, producing approximately 167 million tonnes of aggregates annually as of 2023 [2]. In addition, more than 8000 Legacy pits have been decommissioned and relinquished to “The Ontario Aggregate Resources Corporation” [2]. Sites are rehabilitated at a rate of about 13 per year, and at this rate, it will take about 400 years to rehabilitate all Legacy pits. In 1971, the need for rehabilitation of these sites was recognized, and legislation was implemented, stipulating that every site be rehabilitated to conditions compatible with neighboring lands [3]. However, most sites remain without rehabilitation.

1.1. Current State of Decommissioned Quarries and Mines

In Ontario, decommissioned and abandoned aggregate and mining sites, which began operation before requiring extraction licensing, are now considered Legacy pits. Legacy pits are rehabilitated with the landowner’s permission, transitioning the site into a functional part of the landscape. There are three drivers for the rehabilitation of decommissioned quarries: the economy, biodiversity, and a combination of the two, which results in an area with biodiversity and economic value [4]. Commonly, aggregate pits are rehabilitated into natural sites (25%) or open spaces (15%) [5], such as parks for community use.

The current rehabilitation processes emphasize afforestation or transforming the site into landscapes such as ponds, golf courses, or residential areas, which offer various ecosystem services [4,6]. Rehabilitation typically involves several key processes, including plowing, applying herbicides, and adding a topsoil layer across the entire area [7]. These practices are essential for restoring degraded landscapes’ ecological health and functionality. A variety of vegetation is selected for transplantation to the site based on their resilience, winter tolerance, adaptability to the topsoil conditions, suitability to the proposed developmental site, and ability to re-establish [7,8,9]. Lowe (1979) [7] suggested that of the 60 species planted during the rehabilitation program in Ontario, only 19 survived, presumably due to low nutrient concentration and high soil pH. More recent studies suggest that natural restoration of vegetation through succession may be possible if the vegetation surrounding the site is indigenous [10].

1.2. Toxins and Their Impacts on Environmental Health

According to the Ontario Stone, Sand, and Gravel Association [5], aggregate materials are extracted without chemicals. However, some studies have suggested otherwise [11]. The removed overburden material is kept onsite for replacement during the site rehabilitation process [5]. Operations in quarries and mines involve using machinery and explosives to extract and transport aggregates. The process begins with machinery that removes the topsoil to reveal the underlying aggregate. Explosives are used to break apart and dislodge the desired materials [8]. Potentially toxic elements (PTE) leach out from the weathered rocks. Additionally, the use of petroleum, lubricants, oils, dynamite, and wastewater sludge leaves residuals, and the combustion emissions leave heavy metals and trace elements on the surface soil [12,13,14]. These activities leave behind various pollutants, causing negative impacts on the ecosystem, including contamination of soils and surface, and groundwater. In addition, the process of washing or rinsing the aggregate has also been found to cause seepage of organic substances, nitrates, and heavy metals [15]. High concentrations of contaminants and the oxidative stress from free radicals render soils unsuitable for plant growth [15].

Soil samples from the quarry sites in Southeastern Nigeria show elevated levels of Co (cobalt), Cr (chromium), Nb (niobium), Sn (tin), Ni (nickel), Th (thorium), V (vanadium), and Zn (zinc) [16]. In fact, Cr, Ni, and Co contaminations are common in quarry sites along with Ba (barium) and Pb (lead) [16]. In addition to the elevated levels of heavy metals, deficiencies of P (phosphorus), N (nitrogen), Ca (calcium), Mg (magnesium), and Mn (manganese) were reported following quarry extraction [17,18,19]. A comparison of excavated sites to undisturbed sites in Finland showed that carbon dioxide, nitrate, sulfate, and chloride concentrations were higher in the excavated sites [15]. If not remediated, the contaminated residuals can lead to health issues for those who use the property. Additionally, soil and water pollution reduce the ability of plants to grow in these areas [20].

Traditional remediation techniques, chemical and physical methods, that are often used in contaminated ecosystems, are implemented with significant consequences. Secondary pollutants are introduced with chemical remediation, and soils composition is transformed, leading to further degraded systems [21,22]. Physical methods, such as excavation and soil washing, are costly and further disturb the environment, requiring extensive labor and infrastructure.

1.3. Current Research on Phytoremediation in Quarries

Recent studies on quarries and mines have concentrated on rehabilitating decommissioned sites into valuable habitats; therefore, natural succession has become less of a focus. Research to date has focused on two methods of site rehabilitation. The first provides additional substrate materials to support the natural growth of plants and thus facilitate natural succession. The second pertains to planting new plant species to immobilize or sequester contaminants. Munford et al. (2022) [23] found that native tree species tolerant of heavy metals and acids can facilitate the growth of less-tolerant species like conifers. This illustrates how native plants facilitate the remediation of soil contaminants from quarry and mining sites.

1.4. Phytoremediation

Many plants that grow spontaneously in brownfield conditions have phytoremediation qualities and may be useful in rehabilitating decommissioned quarry sites [12,24]. Phytoremediation involves absorbing, translocating, and storing heavy metals in the plant biomass [25], even though they are not required for plant growth [26]. Plants that remediate are classified as either tolerant or hyperaccumulators of pollutants, meaning that they show no signs of toxicity or rapid growth and store pollutants in their roots and shoots [27]. Metallophyte plants can tolerate high levels of heavy metals. These plants may be obligate, i.e., they survive only in the presence of the metals, or facultative, i.e., they tolerate the metals but do not require them for growth [28].

Plants exhibit several mechanisms to cope with heavy metal pollution. Species categorized as excluders resist the absorption and accumulation of heavy metals [27]. The metal chelating agents released by the microorganisms in the rhizosphere help to prevent heavy metal absorption in plants [27]. The other group of plants categorized as hyperaccumulators uses biochemical pathways to maintain the absorbed metals in their cytoplasm, thereby protecting the other cell organelles from toxicity. However, plants that accumulate large amounts of heavy metals but with no prevention strategy, which sequester or compartmentalize the metals in their vacuoles, so that they are kept away from the cytosol to reduce the damage [27]. Other detoxification mechanisms include the secretion of organic solutes and amino acids, which can translocate the heavy metals to other parts of the plant. Plants that can be used as indicator plants uptake more heavy metals from the soil than others [29]. In summary, plants deal with metal accumulation by complexation, compartmentalization, or metabolic adaptation of the absorbed metals [30,31,32]. The ability of heavy metal remediation by a plant depends on the combination of metals, plant species, and the metal solubility. The metals, after absorption, pass through the epidermis and endodermis to the xylem, where it is translocated to the other parts of the plant [27]. The following are the major phytoremediation processes exhibited by plants.

Phytoextraction: Hyperaccumulators can uptake large amounts of heavy metals from the soil, translocate, and sequester them quickly in the body [33]. In terrestrial phytoremediation projects, this is the most desirable and efficient cleanup method. It involves the accumulation of heavy metals in the upper biomass of the plants, mainly in the shoots [27]. Plants with deep roots and large biomass are best suited for phytoextraction [34]. This method of remediation is particularly useful in locations where the upper biomass can be harvested and subsequently removed from the site. If the biomass is not collected, the plants lose their effectiveness in the remediation process, as the decay of fallen leaves and stems leads to decomposition and the release of pollutants into the environment.

Phytodegradation: This is a process in which plants metabolize pollutants, converting them into less toxic forms and redistributing these less harmful compounds throughout their tissues. A related process, rhizodegradation, occurs in the rhizosphere, where contaminants are broken down and released into the soil with the help of microorganisms [29,35].

Phytovolatization: This process refers to the ability of plants to uptake pollutants and convert them into less harmful forms, which are then released as gases through stomata or lenticels, or within the soil–root interaction zone. This highlights the role of plants in environmental remediation [29]. Few natural plants can transform heavy metals, such as selenium and arsenic, into gaseous states and release them into the environment [36]. Although there are concerns that plants may emit transformed pollutants, such as volatile compounds, strategies for mitigation include microbial assistance via soil amendments and ongoing monitoring [37].

Rhizofiltration: In this process, plants absorb pollutants through ground or surface water, where they are concentrated and precipitated in the roots and other metal-tolerant organs. Endophytic bacteria support this process [29]. Aquatic plants commonly use this method of phytoremediation, which is recommended for cleaning up highly eutrophic waters. Plants with fibrous roots that have large surface areas are particularly effective for this purpose, as their roots function like pumps to draw in water [36].

Phytostabilization: Plants store pollutants in the roots or rhizosphere and reduce their mobility and bioavailability in the water and soil [29,33]. This process occurs in the vadose layer [36], where the pollutants bind to the roots [36]. Some plants can prevent further poisoning or remobilization of the pollutants by forming residues with the help of proteins and phytochemical exudates [36,38]. Plants that use either phytostabilization or rhizofiltration have low transfer factor values and are not recommended for phytoremediation [39].

Compared to conventional methods, phytoremediation offers lower economic costs due to minimal infrastructure and energy inputs, and the recovery of biomass can provide revenue after harvesting [40]. However, it should be noted that although a sustainable solution, its application and success can vary due to biogeochemical factors such as soil type, climate, and other factors, including the depth of the contamination [40].

The literature review revealed a significant gap in information regarding phytoremediation for the removal of contaminants in decommissioned pits and quarries, which encompasses critical aspects such as species selection and efficacy, mechanisms involved in the uptake of contaminants, interactions of multispecies within a site, and assessment of ecosystem recovery. Furthermore, there is limited insight into the phytoremediation processes related to contaminant removal arising from aggregate extraction. Consequently, this study seeks to explore the potential of the spontaneous succession of indigenous and naturalized plants in the phytoremediation of contaminants from closed pits and quarries in Ontario.

2. Methods

A comprehensive search strategy was employed to examine previous studies on the efficiency of native and naturalized plants in the phytoremediation of decommissioned quarry sites in Ontario, Canada. Electronic databases such as Google Scholar, PubMed, and OMNI were utilized to search for relevant articles using keywords including phytoremediation, spontaneous vegetation, decommissioned quarries and mines, brownfields, site conditions, rehabilitation, contaminants, explosives, emissions, extraction, aggregates, native species, and plant colonization. The focus was primarily on articles from Ontario, Canada, and North America. Additional literature searches were performed to determine which toxins may be present in a decommissioned site and the conditions of the soils after the spread of overburden. Plants were categorized by their mechanism of phytoremediation, woody or herbaceous species, and the contaminants they were found to remediate. A Floristic Quality Index Assessment (FQA) was used to gauge the ecological integrity of the site/region. FQA has been widely used, especially in North America, as a community-based habitat quality assessment measure [41]. The FQA tool was developed in 1977 by Gerould Wilhelm [42]. This system assigns a coefficient of conservatism value (C-value) to each plant species in a region. The C-value for a plant species can range from 0 to 10, with 10 indicating very low tolerance to environmental degradation and 0 representing the highest tolerance. Non-native plants are assigned a C-value of 0. The C-value of a plant species can vary from one region to another [41]. The mean C-value of plant species present in an area, along with the species richness, is used in the calculation of the FQA of a particular region. The calculation of C-values helps determine whether a plant species can grow and survive in a disturbed area. Therefore, if a plant can survive and grow in a degraded area, it can be a potential candidate for phytoremediation.

3. Results

3.1. Plant Species Capable of Phytoremediation

This literature review revealed 74 plant species (see Appendix A,

Table A1. Plant Species and phytoremediation information: Plant species growing on decommissioned quarry sites with phytoremediation capabilities. Includes common name, family name, scientific name, and contaminants that can be remediated by the plant, including the mechanism and amount of uptake in mg/kg.

Common Name	Scientific Name	Family	Contaminates	Mechanism of Phytoremediation	Metal Accumulation mg/kg	pH Preference	Suggested for Rehabilitation	References
Alfalfa	Medicago sativa	Fabaceae	Zn, Diesel, V, TNT, PAH, Hydrocarbon, HMX, Pb, Cu, Cr, As	V, TNT, PAH–Phytostabilization; Diesel, Cu, Cd,	Pb-43,300	6.8	no	[98,99,100,101,102,103,104,105,106]
Amur Silvergrass	Miscanthus sacchariferous	Poaceae	RDX, Zn, Cr	Phytoextraction and Phytostabilization			yes	[107]
Bent grass	Agrostis spp.	Poaceae	Zn, Pb, Cu, As, Cd, PAH, TNT	Phytoextraction			yes	[108,109]
Bermuda grass	Cynodon dactylon	Poaceae	TPH	Rhizodegradation				[110]
Birch	Betula papyrifera	Betulaceae	Hg, Cd, Pb, Zn Ni, As	Phytostabilization	1000		yes	[111,112,113]
Bushgrass	Calamagrostis epigejos	Poeacea	As	Phytostabilization				[114]
Birds Trefoil	Lotus corniculatus	Fabaceae	Zn, PAH, Cd, Cr, Pb, As	Phytostabilization			yes	[114,115,116,117]
Milk thistle	Silybum marianum	Asteraceae	Pb, Cu, Cd, Diesel	Phytoextraction	Zn 270.9, Cd 72.3, Pb 401.8	7.4	no	[118,119]
Fleabane	Dittrichia viscosa	Asteraceae	Cd	Phytoextraction			no	[108]
Canadian Goldenrod	Solidago canadensis	Asteraceae	Pb, Zn, As, Mo, Sb, Pb, Cu, Cr	Phyto stabilize, Phytoextraction	Zn 520, Pb 520		no	[78,120,121]
Cattail	Typha latifolia	Typhaceae	As, Pb, Sn, Cd, Cr, Pb, Zn, Ni, Cu	Phytoextraction, Pb-Phytostabilization		6	yes	[122,123,124,125]
Chicory	Cichorium intybus	Asteraceae	Cr, Cd, Pb	Cd, Pb-Phytoextraction; Cr-Phytostabilization	0	Cd 2.43, Pb 12.25, Cr 1730	no	[126,127]
Chinese mustard	Brassica juncea	Brassicaceae	Pb, Cu, Ni, Hg, Cd, V, Zn, Cr, HMX, Au, Co	Ni-Phytostabilization; Pb, Cu, Hg, Cd, V, Zn-Phytoextraction	Ni-3916, Cr	5.7	yes	[82,128,129,130,131,132,133,134,135]
Common reed	Phragmites australis	Poaceae	Pb, Zn, Cu, Ni, Hg, Cd, Cr, As, HMX, RDX, TNT, V, PAH, Co, Hydrocarbons (with bacteria)	Cu, Ni, Zn, Hg-Phytoextraction; Pb, Cd, As, Co, Ba-Phytostabilization			yes	[123,136,137,138,139,140,141,142,143,144]
Common Chickweed	Stellaria media	Caryophyllaceae	Cd, Zn	Phytoextraction			no	[145,146]
Common Mallow	Malva parviflora	Malvaceae	Zn, Pb, Ni, Cd, Cr, Co	Phytoextraction			no	[147]
Common Pokeweed	Phytolacca americana	Phytolaccaceae	Cd, Zn	Phytoextraction			no	[148,149]
Common ragweed	Ambrosia artemisiifolia	Asteraceae	Pb, Zn, Cd, Cu, Cr	Phytostabilization, Phytoextration	Zn 2332, Pb 305	5.5–7.5	no	[150]
Common Sunflower	Helianthus annuus	Asteraceae	TNT, Hg, Pb, Zn, Cd, Cu, Cr, Ni, PAH	Rhizofiltration; Hg, Zn, Pb-Phtyoextraction		5.3	yes	[131,135,151,152,153,154]
Common water hyacinth	Muscari neglectum	Pontederiaceae	Zn, Pb, Cu, Hg, Cd, Cr,	Rhizofilitration	Hg 5600	all pH	no	[155,156]
Common wormwood	Artemisia vulgaris	Asteraceae	Zn, Cd, Cu	Phytoextraction			no	[145,157]
Common Yarrow	Achillea millefolium	Asteraceae	Pb, Cu, Cd, As, Zn, Hg	Phytoextraction			no	[145,157,158,159]
Creeping Bent grass	Agrostis scarbra	Poaceae	Cd	Phytoextraction			yes	[109]
Pondweed	Potamogeton spp.	Potamogetonaceae	Pb, Cu, Ni, Cr,	Phytoextraction			no	[160,161]
Dandelion	Taraxacum officinale	Asteraceae	Cd, As, Pb, Zn, Ni, Cr, Sr	Phytoextraction			no	[80,118,162,163]
Red-Osier Dogwood	Cornus sericea		Cd, Cu, Pb	Phyto				[164]
Scots Pine	Pinus silvestris	Pinaceae	Diesel, Cd, Pb, Zn	Phytostabilization			yes	[165,166]
Eurasian watermilfoil	Myriophyllum spicatum	Haloragaceae	Co, TNT, Ni, Cu, Zn	Phytoextraction Transformation			no	[167,168]
Fescue	Festuca arundinacea	Poaceae	Cr, Cd, Pb, Zn,	Phytoextraction			yes	[169]
Creeping red fescue	Festuca rubra	Poaceae	Cu, Pb, As	Phytoextraction				[114,170]
Field bindweed	Convolvulus arvensis	Polygonaceae	Cu, Cd, Cr,	Phytoextraction			no	[171]
Fox sedge	Carex vulpinoidea	Cyperaceae	RDX	Phytoextraction			no	[161]
Golden Rod	Solidago canadensis	Asteraceae	Zn, Pb, Mo, Cu,	Zn-Phytostabilization; Pb, As, Mo-Phytoextraction	Zn-450		no	[120,172,173]
Bedstraw	Galium mollugo	Rubiaceae	Zn, Cd, Cu	Phytostabilization	Zn-28.4	5.8	no	[145,174]
Himalayan Balsam	Impatiens glandulifera.	Balsaminaceae	Cd, Cu, Zn, B					[175]
Honey Locust	Gleditid Tricanthos	Fabaceae	Pb, Cd	Phytoextraction			yes	[176]
Kentucky Blue grass	Poa pratensis ssp. pratensis	Poaceae	Pb, Cd, Hg	RDX-Phytostabilization; Zn-Phytoextraction		5.5	yes	[177,178,179,180]
Lambs quarter	Chenopodium album	Chenopodiaceae	Cd, Pb Hg	Phytoextraction			no	[181]
Lesser duckweed	Lemna minor	Lemnaceae	Pb, Cu, Ni, Cd, Cr, Zn, Co, Sr, BPA	Phytoextraction			no	[182,183]
Little Blue stem	Schizachyrium scoparium	Poeceae	PAH					[184,185]
Morning glory	Ipomoea spp.	Convolvulaceae	Pb, Ni,	V-Phytostabilization; Pb, Ni-Phytoextraction			no	[186]
Mullein	Verbascum thapsus	Scrophulariaceae	Pb, As, Cd, Cu, Cr	Phytoextraction	Pb 1840, Zn 7807, Cd 141, As 37.8	6.5–7.8		[187,188]
Narrow-Leaved plantain	Plantago lanceloata	Plantaginaceae	Pb, As	Phytoextraction			yes	[106]
Norway maple	Acer platanoides	Aceraceae	Pb, Cd, Cu,	Phytoextraction		5.28	yes	[189]
Old field Cinquefoil	Potentilla simplex	Rosaceae	Zn, Pb, Cu, Cd	Pb-Phytoextraction, Zn-Phytostabilization			no	[164,174]
Orchard grass	Dactylis glomerata	Poaceae	PAH,	Phytostabilization			yes	[78,190]
Pearly everlast	Anaphalis margaritacea	Asteraceae	As, Al, Zn	Phytostabilization	95		no	[191]
Penny cress	Thlaspi arvense	Brassicaceae	Zn, Cd, Ni	Phytostabilization			no	[192]
Perennial ryegrass	Lolium perenne	Poaceae	Zn, Pb, Cu, PAH, Cd, HMX, Diesel	Zn, Cu, As-Phytodegradation; Pb, Cu, PAH-Phytoextraction		4.5–8	yes	[100,100,118,152,165,193,194,195,196]
Pondweed	Potamogeton pusillus	Potamogetonaceae	Cd, Cr, Cu, As, Pb	Rhizofiltration			no	[144]
Poplar	Populus alba	Salicaceae	Zn, Pb, Cu, As, Diesel, HMX, RDX, TNT, Cd, Sa,				yes	[134,197,198,199,200]
Poplar	Populus balamifera	Salicaceae	As, Cd					[102,134,201]
Rapeseed	Brassica napus	Brassicaceae	Zn, Pb, Cd	Phytoextraction			no	[202,203]
Red clover	Trifolium pratense	Fabaceae	Zn, PAH,	Phytoextraction	80		yes	[195,204]
Red fescue	Festuca rubra	Poaceae	Hg, Diesel, Cu, Cd, Ni, Pb, Zn	Phytoextraction	84		yes	[205,206]
Norway Spruce	Picea abies	Pinaceae	Pb, Zn, Cd	Phytoextraction			yes	[100,166,207]
Small Alyssum	Alyssum	Brassicaceae	Ni, Zn, Co	Phytostabilization			no	[134,208,209]
Scots Pine	Pinus Silvestris	Pinaceae	Cd, Cr Pb, Zn					[166,210]
Smartweed	Persicaria punctata	Polygonaceae	Pb, Cu, Cr	Phytoextraction			no	[211]
Smooth Brome	Bromus inermis	Poaceae	PAH, TNT	Phytoextraction			no	[212]
Spikegrass	Eleocharis acicularis	Cyperacea	Cu, Cd, Zn, As, Pb	phytoextraction				[213,214]
St John’s Wort	Hypericum perforatum	Clusiaceae	Cr, Pb, Cd Ni	Phytoextraction	Cu 6.34, Zn 68.31, Pb 47.51		no	[215]
Switchgrass	Panicum vigratum	Poaceae	Pb, Cr, Cd, Zn, PAH, RDX, Cu	Pb, Cd, Hg, As-Phytoextraction; Cr-Phytostabilization	1543	3.4–6.1	no	[183,184,216,217,218]
Tall Fescue	Lolium arundinaceum	Poaceae	Zn, Pb, Cu, Cd, Diesel, Hg, Ni, PAH	As-Photoextraction; Cu-Phytodegradtion; Zn-Phytostabilization; Pb-Rhizofilitration			no	[205,219,220]
Tamarack	Larix laricina	Pinaceae	Ba, Cd, Co	Phytostabilization	280		yes	[221,222]
Tansy	Tanacetum vulgare	Asteraceae	Zn, Pb, Cu, Cd, Cr	Phytoextraction			no	[150,223]
Tree of heaven	Ailanthus	Simaroubaceae	Pb, Cu, Cd	Phytoextraction	2037		yes	[224]
Trembling Aspen	Populus tremuloides	Salicaceae	Cu, Ni, Zn	Phytostabilization and Phytoextraction				[225]
Tuffed hairgrass	Deschampsia cespitosa	Poaceae	Cu, Cr, Zn	Phytostabilization			yes	[225]
Verbena	Verbena officinalis	Verbenaceae	Cr,	Phytoextraction			no	[194]
Viburnum (awabuki)	Viburnum opulus ssp.	Caprifoliaceae	Pb, Cu	Phytoextraction			no	[226]
White Amaranth	Amaranth	Amaranthaceae	Cu, Pb, Cd, Ni, Cr, Zn	Phytoextraction		6.9–5.5 pH	yes	[227]
White clover	Trifolium repens	Fabaceae	diesel, As, TPHs, Cd, Zn	Rhizodegradation	Diesel 80	7.5	yes	[165,228,229,230,231]
White spruce	Picea glauca	Pinaceae	As	Phytostabilization	3642		yes	[222]
Willow	Salix spp.	Salicaceae	Cu, Ni, PAH, As, Cd, Zn	Phytostabilization		5.0–6.0	yes	[232,233,234]

) that grow in decommissioned quarries in Ontario and have the potential to remediate 20 distinct contaminants (Table A1) found at these sites. These plant species employ a range of mechanisms to sequester and immobilize contaminants effectively.

Among the plant species capable of phytoremediation, 21% were woody species (trees 18% and shrubs 3%). In comparison, the remaining 78% were herbaceous species, consisting of 56% forbs, 19% grasses, and 2% sedges (Figure 1). The plants belonged to 28 families, of which the majority were from the Poaceae (Grasses), Asteraceae (Aster family), Fabaceae (Legume family), and Brassicaceae (Mustard family). The percentage of native to non-native plants is from 45.2% to 54.8%.

An FQA for all plants (data available, 74 species), including naturalized and native species, showed a mean coefficient of 1.9 for all species and 4.4 for native species. The score suggests that the plants growing in the decommissioned quarries are found in many ecosystems and do not suggest they are remnants. This number was then used to calculate the FQI (Floristic Quality Index), which was found to be 16.2 for all species (native and non-native) and 24.1 for the native species (

Table A2. Plants identified in the study have the potential to remediate contaminants often found in decommissioned quarries. The “Coefficient of Conservatism” value is represented by “C”. This number ranges from 0 to 10. Higher values are considered most conservative and are intolerant to disturbances, while lower values indicate tolerant species often thrive on disturbed or degraded land. The wetness value, indicated by the “W”, is a measure of the tolerance to moisture levels. Plants with negative values prefer moist locations.

Scientific Name	Family	Acronym	Native?	C	W	Physiognomy
Acer platanoides	n/a	ACEPLAT	non-native	0	5	tree
Achillea millefolium ssp. lanulosa	n/a	ACHMILLL	native	0	3	forb
Agrostis perennans	n/a	AGRPERE	native	5	1	grass
Agrostis scabra	n/a	AGRSCAB	native	6	0	grass
Ailanthus altissima	n/a	AILALTI	non-native	0	5	tree
Amaranthus albus	n/a	AMAALBU	non-native	0	3	forb
Ambrosia artemisiifolia	n/a	AMBARTE	native	0	3	forb
Anaphalis margaritacea	n/a	ANAMARG	native	3	5	forb
Artemisia vulgaris	n/a	ARTVULG	non-native	0	5	forb
Betula papyrifera	n/a	BETPAPY	native	2	2	tree
Brassica juncea	n/a	BRAJUNC	non-native	0	5	forb
Brassica napus	n/a	BRANAPU	non-native	0	5	forb
Bromus inermis ssp. inermis	n/a	BROINERI	non-native	0	5	grass
Calamagrostis epigejos	n/a	CALEPIG	non-native	0	0	grass
Camassia scilloides	n/a	CAMSCIL	native	10	−1	forb
Carex vulpinoidea	n/a	CARVULP	native	3	−5	sedge
Chenopodium album	n/a	CHEALBU	non-native	0	1	forb
Cichorium intybus	n/a	CICINTY	non-native	0	5	forb
Convolvulus arvensis	n/a	CONARVE	non-native	0	5	forb
Dactylis glomerata	n/a	DACGLOM	non-native	0	3	grass
Deschampsia cespitosa ssp. cespitosa	n/a	DESCESPC	native	9	−4	grass
Festuca arundinacea	n/a	FESARUN	non-native	0	2	grass
Festuca rubra	n/a	FESRUBR	non-native	0	1	grass
Galium mollugo	n/a	GALMOLL	non-native	0	5	forb
Gleditsia triacanthos	n/a	GLETRIA	native	3	0	tree
Helianthus annuus	n/a	HELANNU	non-native	0	1	forb
Hypericum perforatum	n/a	HYPPERF	non-native	0	5	forb
Impatiens glandulifera	n/a	IMPGLAN	non-native	0	−3	forb
Ipomoea pandurata	n/a	IPOPAND	native	9	3	forb
Larix laricina	n/a	LARLARI	native	7	−3	tree
Lemna minor	n/a	LEMMINO	native	2	−5	forb
Lobularia maritima	n/a	LOBMARI	non-native	0	5	forb
Lolium perenne	n/a	LOLPERE	non-native	0	3	grass
Lotus corniculata	n/a	LOTCORN	non-native	0	1	forb
Malva neglecta	n/a	MALNEGL	non-native	0	5	forb
Medicago sativa	n/a	MEDSATI	non-native	0	5	forb
Miscanthus sacchariflorus	n/a	MISSACC	non-native	0	5	grass
Muscari botryoides	n/a	MUSBOTR	non-native	0	5	forb
Myriophyllum spicatum	n/a	MYRSPIC	non-native	0	−5	forb
Panicum virgatum	n/a	PANVIRG	native	6	−1	grass
Phragmites australis (P. communis)	n/a	PHRAUST	native	0	−4	grass
Phytolacca americana	n/a	PHYAMER	native	3	1	forb
Picea abies	n/a	PICABIE	non-native	0	5	tree
Picea glauca	n/a	PICGLAU	native	6	3	tree
Pinus strobus	n/a	PINSTRO	native	4	3	tree
Pinus sylvestris	n/a	PINSYLV	non-native	0	5	tree
Plantago lanceolata	n/a	PLALANC	non-native	0	0	forb
Poa pratensis	n/a	POAPRAT	native	0	1	grass
Polygonum achoreum	n/a	POLACHO	native	0	5	forb
Populus alba	n/a	POPALBA	non-native	0	5	tree
Populus balsamifera	n/a	POPBALS	native	4	−3	tree
Populus tremuloides	n/a	POPTREM	native	2	0	tree
Potamogeton alpinus	n/a	POTALPI	native	10	−5	forb
Potamogeton crispus	n/a	POTCRIS	non-native	0	−5	forb
Potamogeton pusillus	n/a	POTPUSI	native	5	−5	forb
Potentilla simplex	n/a	POTSIMP	native	3	4	forb
Salix alba	n/a	SALALBA	non-native	0	−3	tree
Salix pyrifolia	n/a	SALPYRI	native	10	−4	shrub
Schizachyrium scoparium (andropogon s.)	n/a	SCHSCOP	native	7	3	grass
Solidago canadensis	n/a	SOLCANA	native	1	3	forb
Stellaria media	n/a	STEMEDI	non-native	0	3	forb
Tanacetum vulgare	n/a	TANVULG	non-native	0	5	forb
Taraxacum officinale	n/a	TAROFFI	non-native	0	3	forb
Thlaspi arvense	n/a	THLARVE	non-native	0	5	forb
Trifolium pratense	n/a	TRIPRAT	non-native	0	2	forb
Trifolium repens	n/a	TRIREPE	non-native	0	2	forb
Typha latifolia	n/a	TYPLATI	native	3	−5	forb
Verbascum thapsus	n/a	VERTHAP	non-native	0	5	forb
Verbena simplex	n/a	VERSIMP	native	9	5	forb
Viburnum opulus	n/a	VIBOPUL	non-native	0	0	shrub

). These low scores suggest a low quality of conservatism for all species, which means that the plants growing in the decommissioned quarries are mixed populations of native and non-native species and are resilient and quite tolerant to the degraded conditions. The plant communities observed in decommissioned quarries are characterized by resilience and opportunism, exhibiting a mixed composition that has adapted effectively to the specific environmental conditions present within these settings. Notably, these communities are devoid of species sensitive to environmental changes or serve as indicators of higher-quality habitats. This adaptability highlights the unique ecological dynamics in disturbed environments such as quarries.

3.2. Remediation Abilities of Plant Species

The number of contaminants that each plant species could remediate is presented in Figure 2. The study concluded that, out of the 74 plant species examined, only one species, Phragmites australis, could remediate 12 contaminants. Populus alba and Brassica juncea, followed by remediating 10 contaminants. A total of 84% of all the plant species studied could remediate more than one contaminant, while 10 plant species, representing 13% of the total, could remediate only one contaminant.

3.3. Contaminate vs. Plant Species

The number of contaminants that a plant species can absorb or remediate is illustrated in Figure 3. Among the 74 plant species assessed, 48 were capable of remediating cadmium (Cd) either in the upper biomass (27 species) or in the lower biomass (21 species). In comparison, lead (Pb), zinc (44), copper (Cu), and chromium (Cr) were remediated by a smaller number of species, ranging from 44 to 25. Interestingly, tin (Sn) was unique in that only one plant, Typha latifolia, was identified as capable of remediating it.

Finally, the phytoremediation mechanisms utilized by plant species were assessed (Figure 4). Among the woody species (n = 16), contaminants were primarily stored in the lower biomass by phytostabilization (53% of the time) and rhizofiltration (5% of the time). In contrast, herbaceous species (n = 58) tended to store contaminants in their upper biomass by phytoextraction (58% of the time) and phytodegradation techniques (1% of the time).

4. Discussion

4.1. Scarring the Landscape Through Excavation

Quarry and mining operations begin with clearing the overburden material and extracting the desired material as required and permitted. Explosives are often required to loosen the material for removal, which is loaded and transported to the crushers and the screener [4]. Once extraction is no longer feasible, the quarry or pit is decommissioned. At this stage, the ecological and economic value of the area has been diminished, resulting in a scarred and abandoned landscape [1]. Due to the vast number of abandoned quarries and mines waiting for rehabilitation in Ontario, the Government now grants new operation/extraction licenses only if a rehabilitation plan is included as part of the proposal. Unfortunately, the Legacy pits are still left without rehabilitation plans.

4.2. Damage to Biodiversity

One of the most significant impacts of the extraction of aggregates is the damage to biodiversity. Stripping the overburden material using explosives and dumping the extensive volumes of waste ultimately leads to chemical and mineral runoff, contaminating the nearby soils and water bodies. Undesired trace elements and toxic chemicals in the soils and waterbodies can have reduced growth, impaired reproduction, and decreased biodiversity [43]. Excavation activities further deteriorate habitats by decreasing water quality, altering water flow, and increasing erosion. Succession is influenced by its surrounding vegetation and soil texture, soil properties, and water availability [44]. The moisture gradient aids in nutrient exchange and water uptake, making it one of the most important landscape factors influencing vegetation [10]. Soil textures influence water and nutrient availability conditions, therefore affecting plant species’ establishment [44]. The pH of the soil can vary depending on the bedrock type; for example, in the case of limestone quarries, the pH would be alkaline [44]. Low soil pH is linked to an increased availability of metal ions [14]. However, if the pH is low, the ability of microbes to fix nitrogen is hindered, resulting in lower degradation of organic matter [14,45]. The literature review revealed that quarry pH levels vary between 5.1 and 8.98 (

Table 1. pH and nitrogen (TKN) as identified in studies conducted by several authors, n = the number of quarry sites in each study.

	Gladys [46] n = 1	Lago-Vila [13] n = 3	Tomlinso et al. [47] n = 65	Rani et al. [48] n = 32	Rodriguez-Seijo et al. [49] n = 1	Recommended Values
pH	8.98	5.99–7.87	7.68	5.1–8.8	7.87–8.05	6.3–7.5
TKN	0.005	0.03–2.75	4.96	8.1–113	0.042–UL *	0.2–0.5

* UL undetectable level.

).

4.3. Importance of New Habitat Development

Old quarries and pits are often viewed as wastelands. However, these semi-barren areas have high biodiversity and provide indispensable habitats for many species, often having higher biodiversity than alvars [47]. These vast open areas provide distinct habitats through variations in slope, soil textures, compaction, nutrients, pH, and drainage [50]. The diversity within the quarry sites supports generalist and specialist species by providing a mosaic of microhabitats [51,52]. In addition, they function as a link or corridor connecting other habitats by supporting invertebrate species movement between them. Nutrient-poor, contaminated soils exhibit a slow succession of species. Generally, these disturbed habitats are colonized by ruderal species, which are highly adaptable to disturbances with shorter life spans and capable of producing an abundance of small seeds [53]. Other substrates, such as rubble and sand within the quarry sites, can provide microhabitats for organisms by providing shelter and space for burrowing and nesting.

4.4. Spontaneous Succession of Native Plants in Abandoned Quarries

Rehabilitation can begin through one of two methods: spontaneous succession from local seed sources or through technical measures, which include leveling, the spread of overburden materials, sowing seeds, and planting various vascular species [54]. Quarry rehabilitation often happens after a long delay, and spontaneous growth of natural vegetation occurs during this time. Natural succession often leads to highly valuable vegetated areas or wetlands [10,55]. This has been demonstrated as a slow but efficient way to restore a disturbed ecosystem [10,39]. Novak and Konvicka [56] illustrated that natural, spontaneous succession is possible when there is a native species pool within 100 m of the site [45]. As the site becomes vegetated, nutrients are brought to the soil surface, and organic matter is accumulated [45]. Nearly all decommissioned sites can recover spontaneously [45]. However, a mere 1.5% are considered for spontaneous succession during planning [45,55]. Conversely, spontaneously restored brownfields are considered a substitute habitat for rare and threatened species and plant communities [1,19,55]. The spontaneous primary ecosystems are important as early successional stages of a community build-up [19,55,56]. Furthermore, Reinhardt et al. [57] suggest that site remediation is most successful when left undisturbed. Re-disturbing the site for rehabilitation can impact the species already established. Moreover, spontaneous succession has been found to establish at the same rate as technical planting, requiring little cost and intervention [54].

The plants examined in this research are typically found in disturbed suburban areas in Ontario. These species are particularly resilient and well-adapted to disturbed and harsh environmental conditions, such as varying levels of soil compaction, contaminants, nutrients, and pH [53]. This resilience makes these natural species the ideal candidates for ecosystem restoration after quarry operations. Studies on spontaneous succession in decommissioned quarry sites have shown that the species composition witnesses gradual changes with time, and with the major changes occurring within the first six years [58]. Many undesirable annual synanthropic species appear in the early stage of succession; however, these are often choked out by other woody-type species. This leads to the establishment of a natural wildlife habitat that can foster a more sustainable ecosystem with the region’s native flora. Thus, the natural seed bank and spontaneous naturalization lead to the restoration of decommissioned quarry sites.

Parch et al. [59] in their study on spontaneously naturalized decommissioned sites in the Czech Republic discovered that over time, the biodiversity of the sites surpassed that of the technically planted sites. Plant biodiversity of naturally colonized decommissioned mines is unique and different from the undisturbed areas [58]. In typical temperate regions, the natural succession follows the order of appearance of annuals, followed by biannually flowering plants and perennial forbs. In a study of mining sites, Wiegleb and Felinks [60] found that wind was the primary mechanism for seed dispersal from local sources, with zoochory being a secondary method. Although invasive species proliferated during the early stages of succession, they were later declined and were replaced with desired local species. This progression is followed by the development of shrublands and finally, the [61] resulting in various vegetation types [60].

The replacement of the overburden material, which was stripped and stored for usage after the pit’s closure, is advantageous and economical. The soil contains many needed nutrients and will help adjust the pH. The replacement of the overburden material eliminates the purchase of topsoil but provides pre-existing organic matter and seed source. The native and pre-existing species provide a foundation for plant succession [62]. Whereas the introduction of new soils, which is often used for technical planting, introduces a change in soil quality, fertility, and ecological integrity [63]. Introduced soils can carry new contaminants and invasive species.

The pH of soils in decommissioned quarries ranges from acidic to alkaline (Table 1), which can support most plant species capable of phytoremediation. Studies on the spontaneous colonization of gravel pits in Quebec and Europe reported slow growth [10,54]. On the contrary, other studies have found that environmental variables did not affect plant species composition [62], allowing ruderal generalist species, such as graminoids, to colonize during the younger seral sites [10].

Contaminant uptake is highly influenced by soil pH. Low acidic soil pH has been shown to increase heavy metal availability [14]. Contaminants such as Pb, Zn, TF, Cu, Co, Ni, and Hg are more soluble at acidic pH, resulting in higher plant uptake, unlike arsenic (As). However, the soil composition, nutrients, and pH change rapidly with the accumulation of vegetative matter [64] and this may result in the colonization of new plant species and the uptake of contaminants.

4.5. Phytoremediation of Contaminated Quarry Sites

Spontaneous regeneration should be considered a method for the rehabilitation and phytoremediation of pits and quarries in Ontario. This review identified 74 plant species capable of taking up 20 contaminants found at the quarry sites (Figure 2; Table A1). Furthermore, studies by Alifragki et al. [65] and Panz and Miksch [66] confirmed that plants can remove explosives and other contaminants from the soil. Plants considered hyperaccumulators can accumulate high levels of heavy metals in their biomass and remain healthy [67]. Plants which occur naturally in metalliferous soils uptake contaminants through their root system and sequester them away from the active metabolic plant cells [68]. These hyperaccumulator plants can be harvested, removing the contaminants from the environment, which can then be used for recovery through a process called phytomining [69], whereby the plants are harvested and burned to release the heavy metals in the form of ash. The ash is collected, and the metals are extracted. Plants can be harvested several times during a season and over several years until the desired reduction in soil contaminants is achieved [68].

As mentioned above, phytostabilization renders pollutants biologically unavailable; therefore, monitoring may be required [70] to ensure that pollutants are not released during times when biogeochemical changes occur, such as shifts in pH or oxygen. Recent studies have found that shifts in nitrogen-fixing bacteria, caused by heavy metals, can alter the soil and influence uptake [71]. On older, long-time vacant sites, where trees are well established, subsequent rehabilitation processes may consider leaving the trees intact to prevent the release of the sequestered contaminants in and around the roots. Alternatively, growing young herbaceous species capable of phytoextraction is recommended. Younger plants, including those in primary succession, can uptake more contaminants than older plants [72].

Phytoremediation has many advantages over other forms of contaminant removal. In large-scale applications, such as quarries, phytoremediation provides an advantage in that it can be applied in situ, at low cost, with low time commitment. It does not require any additional machinery or chemical treatments and, therefore, is a cheaper approach than other forms of remediation [73,74,75]. Furthermore, the practice is socially accepted and considered environmentally friendly.

This review revealed that many plant species can remediate more than one contaminant, except tin (Sn) (Figure 3). Multiple species capable of remediating more than one contaminant are beneficial since seed bank resources limit sites. For initial colonization to occur, the primary source of seeds should be within range of the site. Not every plant identified through this review may be within the vicinity. For example, 48 plants identified in this review could remediate cadmium (Cd), 44 plant species could remediate lead, and 42 plant species could remediate zinc, providing a wide variety of options for remediation. Lead, mercury, and cadmium are the most significant contaminants because of their toxicity and prevalence in the environment [76], and therefore, they may have been studied more in-depth.

This review highlights the limitations of spontaneous phytoremediation. Specifically, for contaminants like diesel, only six plant species have been identified as effective for this process. Consequently, remediation efforts may be hindered if the local seed source lacks these species. However, it is possible that the contaminant can still be addressed later when seeds from these species are introduced through natural dispersal mechanisms.

The number of contaminants each plant species can remediate varies from 1 to 12 per plant (Figure 2). Therefore, remediation can benefit if a mosaic plant distribution within a site is achieved. The greater the number of contaminants a plant can remediate, the better and more efficient the process will be, as a limited number of species can remediate more contaminants.

4.6. Native vs. Technical Plantings

Plants which naturally grow on a site are the best-adapted ones. Plants can only colonize spontaneously when site conditions favor their seed dispersal, growth, allelopathy, and competitive interaction [77]. The benefits of native plants over technically grown plants for phytoremediation are numerous. In Ontario, native plants are more adapted to poor nutrient conditions [78,79] and the harsh cold temperatures compared to the fast-growing commercial species. Additionally, there is no danger of creating new ‘weeds,’ as might happen with the introduced species [79]. Seeds from native species will have genetic variability, permitting survival through evolution to resist environmental stressors [45]. Sustainable phytoremediation permits natural colonizers to grow spontaneously on a degraded landscape [52,80]. Plants which colonize naturally are capable of phytoremediation and may be just as prosperous or even more successful than manually planted commercial varieties.

Plants that naturally grow on a site are best adapted for the climate, the soil and nutrient composition, and the topography. Plants exhibit distinct morphological traits, including tap roots that facilitate improved absorption of water and nutrients. Additionally, many species have adapted by reducing their overall size to conserve resources, while others feature thickened leaves and cuticles to enhance water storage and withstand environmental stress. Some also possess thickened midribs that aid in storing and transporting nutrients. Thus, it would be reasonable to say that plants which naturally colonize a decommissioned mine or quarry are best suited to the soil at the site [81,82]. Metallophyte plants have previously been used as indicator plants for reclaiming industrial sites [83]. They provide high amounts of biomass and thrive on contaminated sites [84,85]. Colonized plants may also contribute to succession by acting as nurse plants, providing canopy cover [86,87], improving microclimate and supplying organic matter through leaf litter, nutrients, etc. [88,89].

Native perennial species grow slowly compared to commercial species, which are designed for fast growth to meet consumer needs. However, slow growth could be advantageous when growing in contaminated soils, enabling them to survive because of slower uptake of toxic elements, which allows them to translocate and sequester in their body [88,89]. In addition, the introduction of hybrid or potentially invasive hyperaccumulator commercial plant species should not be considered because the introduced species might alter indigenous floral diversity [73]. In instances where commercial plants have been used for phytoremediation, it has been found that the spontaneous growth of native plants increased the growth of less tolerant trees with low seed production [23]. Sites that are manually planted have significant upfront costs and time commitments. Further maintenance includes, but is not limited to, irrigation, replacement of lost plants, and the addition of substrates and fertilizers.

4.7. Plant Morphology

Research suggests that plant morphology may influence absorption, transportation, and sequestration. Variations in root lengths can influence plants’ ability to access these metals due to the increase in surface contact area. Studies on plant traits, such as root size, type, and root depth, indicate that differences in metal concentration in plants may be influenced by root depth [90,91]. Furthermore, roots can modify the rhizosphere by altering the availability and solubility of contaminants. This can be seen in wetland plants, which release oxygen through the roots and can alter the chemistry of the rhizosphere [92]. This process can lead to the formation of iron plaques on the roots, which may inhibit or enhance the uptake of metals [93]. Additionally, transporter proteins in the roots play a crucial role in determining which contaminants the plant can absorb [94].

The effectiveness of plants in remediation is largely attributed to their resilience. To thrive in challenging environments, such as degraded decommissioned quarries in temperate areas such as Ontario, Canada, plants must possess cold tolerance and stress resistance. Activated molecular pathways that enhance their ability to endure low temperatures enable these plants to continue absorbing pollutants even during colder periods [95]. Plant resistance inducers aid plants to survive in cold and to balance redox conditions [96].

5. Conclusions

Using phytoremediation to remove contaminants from decommissioned quarries is a sustainable, environmentally friendly method. Several harvests of the plant’s upper biomass may be required to sufficiently remove the toxins from the area. The spontaneous naturalization of native plants capable of growing on the site is a successful remediation technique to remove contaminants. Over two-thirds of the plants, as detected in this review, were able to remediate more than one contaminant, which is beneficial given the heterogeneity and the complex nature of contaminants in a decommissioned quarry. The variation in plant species, which can efficiently uptake multiple contaminants, provides an excellent opportunity to use them for site remediation. Variations in sequestering mechanisms of contaminants by different plant species provide options to remove the contaminants from the site permanently or temporarily (by making them biologically unavailable). Those contaminants sequestered in the root systems and soil can be monitored in situ or left alone on the site as they are biologically unavailable.

Consideration should be given to removing contaminants through harvest and leaving the site for naturalization. Naturalized quarry sites are comparable to other ecosystems and would provide a much-needed diverse habitat for various wildlife. The naturalized sites would also provide a natural setting for leisure activities and ecological education for the neighboring communities.

Further research of quarries for natural phytoremediation should include investigating other plant species that inhabit the decommissioned quarry sites. More studies on contaminant removal timelines and plant upper biomass harvest rates would assist in rehabilitating decommissioned quarry sites. Field studies are important, as most research on phytoremediation of contaminant soils has been performed in experimentally controlled settings [97].