Assessment of Soil and Groundwater Contamination from Olive Mill Wastewater Disposal at Ben Aoun, Central Tunisia

Abstract

Olive mill wastewater (OMW) contains high organic loads and phytotoxic polyphenols. In Tunisia, OMW is often stored in unlined evaporation ponds. This practice creates a risk of soil and groundwater contamination. This study evaluates the environmental impact of a long-term OMW evaporation pond in the Ben Aoun area, Sidi Bouzid region. The investigation combines wastewater, soil and groundwater sampling with laboratory physicochemical analyses. Three OMW samples (E1 surface, E2 mixed, E3 recent spill) were collected. Three shallow boreholes (0–5 m) were sampled at 20 cm intervals. In addition, three nearby pumping wells were sampled. All samples were analyzed for pH, electrical conductivity (EC), chemical oxygen demand (COD), total and volatile solids, major cations/anions, total nitrogen, total phosphorus and total polyphenols. Results obtained using the Folin–Ciocalteu method are expressed as mg Eq AG L⁻¹ for liquids and mg Eq AG gMS⁻¹ for soils. OMW samples showed high COD (E1 = 48, E2 = 70, E3 = 80 g/L) and polyphenols (E1 = 5, E2 = 9.7, E3 = 14 g/L). Soil profiles inside the pond exhibited increased EC with peak of 15.48 mS cm⁻¹ at 0.4 m depth. Near-surface layers showed low pH and increased organic matter and polyphenols to depths of ~5 m. Groundwater samples collected near the pond contained measurable polyphenols (up to 41 mg/L in the closest well), indicating subsurface migration. Evidence indicates lateral migration of about 20 m and vertical infiltration to a depth of approximately 5 m beneath the pond. The findings demonstrate that unlined OMW evaporation ponds act as a persistent source of organic and phenolic contamination. This poses a potential risk to shallow groundwater.

1. Introduction

Olive oil production is one of the oldest and most significant agro-industrial activities in the Mediterranean Basin. It plays a central socio-economic role, sustaining rural livelihoods and contributing substantially to national economies. Tunisia ranks among the leading global producers and exporters of olive oil, accounting for about 7–8% of the world’s production [1]. Annual national output averages roughly 220,000–250,000 tons, and olive cultivation occupies nearly one-third of the country’s agricultural land. Despite its importance, this strategic sector generates large volumes of by-products. Their management poses serious environmental challenges.

The quantity and composition of OMW vary depending on the extraction process. Traditional batch-press systems produce approximately 400–500 L of OMW per ton of processed olives. Modern continuous systems, particularly the three-phase decanter, generate larger volumes ranging from 500 to 1200 L/ton. In contrast, two-phase systems produce a smaller liquid fraction mixed with solid residues [2]. Regardless of extraction technology, OMW is characterized by a dark color, strong odor, high organic content, and high concentrations of phytotoxic phenolic compounds [3,4,5].

Chemically, OMW is a complex mixture derived from the natural water content of olives and from process water used during oil extraction. It contains large amounts of organic matter, including sugars, proteins, organic acids, and lipids. Suspended solids and mineral salts are also present. Reported COD values typically range from 80 to 200 g/L, while biological oxygen demand (BOD) ranges from 35 to 100 g/L. These values reflect the exceptionally high organic load of OMW [2]. Polyphenolic compounds occur at concentrations of 0.5–24 g/L. They are mainly responsible for the effluent’s toxicity and recalcitrance to biodegradation [4,6]. Major mineral components include potassium, sodium, calcium, magnesium, and phosphate, with average concentrations of 15, 5, 1, 0.4, and 0.8 g/L, respectively [5,7]. These physicochemical characteristics make OMW a highly polluting effluent capable of altering soil chemistry, suppressing microbial activity, and degrading surface and groundwater quality when released untreated.

In Mediterranean countries, uncontrolled disposal of OMW remains a major environmental concern [8,9]. More than 30 × 10⁶ m³ of OMW are produced annually across main olive-oil-producing countries such as Spain, Italy, Greece, and Tunisia. Production occurs mainly during the milling season from November to February [10,11]. Due to the effluent’s complex composition and the high cost of treatment technologies, most OMW is disposed of through direct land spreading or storage in natural or artificial evaporation ponds. These ponds are frequently unlined, permitting wastewater infiltration into the subsurface. Consequently, OMW disposal is often associated with soil salinization, acidification, organic enrichment, and contamination of shallow aquifers [4,12].

Numerous studies have investigated laboratory-scale interactions between OMW and soils. These studies report changes in pH, EC, and microbial dynamics [12]. Other studies have examined valorization and treatment strategies. These include composting, anaerobic digestion, and enzymatic degradation aimed at reducing organic load and phenolic content [13,14,15]. Although such methods show promise, their implementation at an industrial scale remains limited due to economic constraints, seasonal production, and the large effluent volume [16,17]. Consequently, natural evaporation ponds remain the most common and economically feasible method in Tunisia and other Mediterranean countries. However, this practice carries considerable environmental risk, particularly in areas where soils are permeable and groundwater tables are shallow [4].

In Tunisia, most OMW evaporation ponds are located in non-engineered areas. They lack liners or drainage systems and rely solely on natural evaporation under semi-arid climatic conditions [16]. Over time, infiltration from these ponds may alter subsurface geochemistry and degrade groundwater quality. Despite their widespread use, few field-based studies have quantified vertical migration of contaminants and their impact on aquifers under real conditions. This knowledge gap limits the development of effective management and monitoring strategies.

The present study addresses this gap by evaluating the environmental impact of an OMW evaporation pond in the Ben Aoun area, Sidi Bouzid, central Tunisia. The specific objectives are: (1) to characterize the physicochemical composition of OMW stored in the pond; (2) to assess the extent of soil contamination and vertical migration of key pollutants (phenolic compounds, organic matter, and salts); and (3) to determine whether contamination has affected nearby groundwater resources. By focusing on a long-term disposal site, this study enhances understanding of OMW–soil–groundwater interactions. It also supports the development of sustainable OMW management strategies in Tunisia and similar Mediterranean settings.

2. Characteristics of OMW and Management Practices

OMW is produced during the mechanical extraction of olive oil and consists of a dark, malodorous liquid containing dissolved and suspended organic matter, including plant residues, mucilage, sugars, and organic acids [12]. The physicochemical composition of OMW varies depending on olive cultivar, ripeness, climatic conditions, and extraction technique [2,5]. The typical composition is summarized in

Table 1. Presents the typical composition of OMW reported in the literature [4,5,18].

Parameters	Range/Average Value
pH	4.5–5.2
EC (mS/cm)	8–16
COD (g/L)	45–130
BOD (g/L)	35–100
Suspended solids (g/L)	1–9
TS (g/L)	60–120
Mineral solids (g/L)	5–15
VS (g/L)	55–105
Sugar (g/L)	10–80
Pectins, mucilage and tannins (g/L)	3.7–15
Polyalcohols (g/L)	1.1–15
Polyphenols (g/L)	5–24
Fats (g/L)	0.5–10
Organic acids (g/L)	5–10
Amino acids (g/L)	2.8–20
PO42− (g/L)	0.8
Na+ (g/L)	5.37
K+ (g/L)	15.29
Ca++ (g/L)	1.17
Mg++ (g/L)	0.41
Mn++ (g/L)	0.01
Cl− (g/L)	0.27
SO42− (g/L)	0.01

.

The effluent is acidic (pH ≈ 4.5–5.5), has high EC value (>7 mS/cm), and contains elevated levels of suspended solids, residual fats, and polyphenolic compounds [3,4,7]. In the main olive-oil-producing countries, annual OMW generation exceeds 30 × 10⁶ m³ [10,11].

Numerous management and valorization strategies have been tested to mitigate OMW pollution. Controlled agricultural reuse of diluted OMW can improve soil fertility due to its organic and nutrient content [19]. However, excessive application leads to soil salinization, reduced porosity, and phytotoxicity [4]. Other technologies include aerobic and anaerobic digestion [15], fungal biodegradation [13,14], chemical oxidation [20], and adsorption on clay minerals [21]. Despite laboratory success, these methods remain economically unfeasible at an industrial scale [18,22].

Consequently, natural evaporation ponds remain the dominant and most cost-effective disposal method in Mediterranean countries. These systems depend on climatic evaporation but often lack liners or engineered leachate control, allowing OMW infiltration that causes vadose-zone contamination and groundwater deterioration [4]. In addition, they emit unpleasant odors and generate sludge residues after drying.

Given the persistence of this practice and the associated environmental hazards, there is an urgent need for field-based investigations that quantify infiltration dynamics, pollutant accumulation in soils, and impacts on groundwater. The present work provides such an assessment for the Ben Aoun pond, contributing essential data for improving OMW management in Tunisia and comparable semi-arid regions.

3. Materials and Methods

This study involved the collection of OMW, groundwater, and soil samples from the investigated area. Each type of sample undergoes physicochemical analysis to identify key pollution indicators. For soil, a detailed lithological study is also performed to precisely determine the infiltration pathway of OMW through the subsurface. All analytical results are then integrated during the geochemical interpretation phase, allowing the identification of contamination patterns and pollutant migration. This combined approach ultimately confirms the infiltration of OMW and its impact on groundwater quality (Figure 1). Sampling was conducted once at each location during the campaign period.

3.1. Study Area

The study site is located near Ben Aoun, approximately 35 km southwest of Sidi Bouzid city in central Tunisia (Figure 2). Sidi Bouzid is among the country’s major olive-growing regions, contributing about 24% of national olive oil production [1]. Olive cultivation covers nearly 254,000 ha, with roughly 40 million olive trees and an average yield of 700 kg/ha. The region’s economic dependence on olive production has led to a proliferation of oil mills and, consequently, the generation of large volumes of OMW.

3.1.1. Climate and Physiography

The study area experiences a semi-arid Mediterranean climate. It is characterized by hot, dry summers and mild, wet winters. Mean annual rainfall ranges from 250 to 300 mm. Most precipitation occurs from November to March. Mean annual temperatures vary between 16 °C and 19 °C. Potential evapotranspiration exceeds annual precipitation. This condition favors the natural evaporation of stored wastewater. Prevailing north winds have an average velocity of about 2.5 m/s. These winds further enhance evaporation from open ponds [23].

3.1.2. Geological and Hydrogeological Setting

Geologically, the investigated area belongs to the central Tunisian domain. It is mainly composed of Quaternary to Mio–Plio–Quaternary sandy and sandy–clay deposits. These deposits overlie Cretaceous carbonate formations of the Zebbag Formation [24]. The shallow aquifer system is hosted within unconfined sandy layers whose hydraulic conductivity depends on texture and clay content. Groundwater occurs at depths between 5 and 20 m, forming the principal source for both irrigation and domestic supply. The combination of permeable lithology and shallow water table increases the vulnerability of the aquifer to contamination by infiltrating OMW and other surface pollutants.

3.1.3. The Ben Aoun Evaporation Pond

The investigated OMW disposal site is located about 1 km south of Ben Aoun town. It comprises a natural, unlined evaporation pond that has received effluents from nearby olive oil mills for over 15 years. The pond covers an estimated 4000 m², with sandy to sandy–clay substrate facilitating rapid infiltration. Wastewater is discharged through open ditches without pretreatment, and no engineering barriers, leachate collection, or monitoring systems are installed. The surrounding landscape is predominantly agricultural, and several irrigation wells lie within a few hundred meters of the pond. These conditions make the Ben Aoun site a representative example of typical OMW management practices in central Tunisia.

3.2. Sampling of OMW

Given that OMW is a hazardous effluent with high organic and phenolic loads, comprehensive physicochemical analyses were performed on liquid samples collected from the Ben Aoun evaporation pond (Figure 3). Three OMW samples were collected, each representing different degrees of aging and degradation:

• E1: collected from the surface of the eastern basin, which contains the oldest OMW residues;
• E2: a composite sample from both surface and bottom layers of the central basin, representing intermediate-aged OMW;
• E3: collected directly from the southern spillway, corresponding to the most recent discharge from the olive mill.

Samples were transferred into pre-cleaned glass bottles, stored at 4 °C, and transported to the laboratory for analysis within 24 h.

3.3. Analytical Methods for OMW Samples

3.3.1. Physicochemical Parameters

• pH and EC were measured in situ using a WTW Multi Line 3430 multiparameter meter, calibrated with standard solutions at 25 °C.
• Total solids (TS) were determined after drying at 105 °C for 24 h, while volatile solids (VS) and organic matter were measured by calcination at 550 °C for 2 h.

3.3.2. Chemical Oxygen Demand

COD was determined by the dichromate reflux method (ISO 6060) [25]. A known amount of OMW was oxidized with excess potassium dichromate in acidic conditions (148 °C for 2 h) in the presence of silver sulfate catalyst. The remaining dichromate was titrated with ferrous ammonium sulfate.

3.3.3. Total Kjeldahl Nitrogen

Nitrogen was determined following the Kjeldahl digestion method [5]. Organic nitrogen was converted to ammonium sulfate using hot concentrated sulfuric acid (H₂SO₄, ρ = 1.82) and a selenium catalyst (Merck). Released ammonia was distilled into boric acid (20 g/L) and titrated with 0.1 N H₂SO₄.

3.3.4. Phenolic Compounds

Total phenolic compounds were extracted from each OMW sample using ethyl acetate (1:1 v/v) under mechanical stirring for 10 min. The organic extract was analyzed by the Folin–Ciocalteu colorimetric method [5], with 4-methylcatechol used as a calibration standard. Absorbance that reflects the concentration of the target compound present in the sample was measured at 765 nm using a UV–Vis spectrophotometer (Labomed UVD-3200, Labomed Inc., Los Angeles, CA, USA).

3.3.5. Mineral Composition

For major cations (Na⁺, K⁺, Ca²⁺, Mg²⁺), the OMW samples were first ashed and subjected to acid digestion (HCl:HNO₃ = 3:1). Elemental analysis was performed using a flame photometer (Jenway PFP-7 model, Ltd., Staffordshire, UK).

All parameters were compared with those reported for raw OMW in [4,5] to evaluate deviations related to waste aging and pond processes.

3.4. Groundwater Sampling and Analysis

Groundwater samples were collected in March 2019 from three pumping wells (W1–W3) located at varying distances (60–3000 m) from the Ben Aoun OMW pond (Figure 4). All wells are used primarily for irrigation. Before sampling, each well was purged for 10 min to remove stagnant water. Samples were collected in pre-rinsed glass bottles, preserved at 4 °C, and analyzed within 48 h. The following parameters were measured:

• pH and EC, determined in situ with a multiparameter probe (WTW Multi Line 3430, WTW GmbH, Weilheim, Germany);
• Total phenolic compounds, analyzed using the Folin–Ciocalteu method as described above.

3.5. Borehole Drilling and Soil Sampling

To characterize subsurface lithology and assess contaminant migration, three boreholes (I, II, III) were drilled to a depth of 5 m around the OMW pond (Figure 3). The boreholes were spaced approximately 20 m apart and positioned as follows:

• Borehole I: outside and adjacent to the first basin,
• Borehole II: outside and near the second basin,
• Borehole III: inside the second basin.

Continuous core samples were collected every 20 cm along each borehole. Additionally, a background site (control point CP) was sampled approximately 1 km northeast of the pond, where two soil samples were collected. Sample CP20 represents the 0–20 cm depth interval, while sample CP40 corresponds to the 20–40 cm depth interval.

3.6. Soil Analysis

Soil samples were air-dried, sieved (<2 mm), and analyzed for the following parameters:

• pH and EC: measured in a 1:2.5 (w/v) soil–water suspension using calibrated electrodes.
• Organic matter determined by loss-on-ignition at 550 °C for 2 h.
• Total phenolic content: extracted using ethyl acetate (1:1 v/v) and analyzed via the Folin–Ciocalteu method.

These analyses were used to evaluate vertical changes in soil chemistry and the penetration depth of organic contaminants beneath and around the OMW pond.

3.7. Data Validation and Comparison

The geochemical results of the Ben Aoun samples (E1–E3) were compared to reference OMW characteristics reported in [4,5] to identify changes associated with waste aging, evaporation, and infiltration processes. For groundwater and soil, results were interpreted relative to World Health Organization (WHO) [26] standards for irrigation and drinking water quality and to soil contamination limits [27].

4. Results and Discussions

4.1. Physicochemical Characteristics of OMW

The physicochemical properties of the OMW samples (E1, E2, and E3) collected from the Ben Aoun evaporation pond are presented in

Table 2. Physical–chemical properties of OMW samples collected from the Ben Aoun pond. Each value corresponds to a single sample collected at the indicated point (E1, E2, E3). Data from [5] are provided for comparison.

Parameters	OMW Samples
	E1	E2	E3	OMW After [5]
pH	4.5	6	5.2	4.5–5.2
COD(g/L)	48	70	80	45–130
Total organic carbon (g/L)	12.5	15.85	110	-
TS (g/L)	50	34	80	60–120
VS (g/L)	44	25	60	55–105
Fats (g/L)	0.5	0.3	0.8	0.5–10
Polyphenols (g/L)	5	9.7	14	5–24
Total N (g/L)	2.8	1.5	6.8	-
C/N	5.5	12.5	12.5	-
Total P (g/L)	0.096	0.45	1.1	0.8
K (g/L)	5.2	5.364	7.2	15.29
Na (g/L)	0.12	0.636	0.7	5.37
Ca (g/L)	0.045	0.85	0.9	1.17
Mg (g/L)	0.1	0.164	0.2	0.41
Fe (mg/L)	35	60	50	-

and compared with the values reported by [5].

The pH values of the analyzed OMW (4.5–6.0) confirm the slightly acidic nature characteristic of untreated effluents. The slightly higher pH of sample E2 is attributed to partial neutralization resulting from interaction with carbonate-rich sediments during storage. COD values range between 48 and 80 g/L, which is consistent with the range of raw OMW (45–130 g/L) reported by [4,5]. The lower COD of E1 (48 g/L) reflects partial biodegradation of organic matter in the older surface layer, whereas the high COD of E3 (80 g/L) indicates freshly discharged effluent with limited oxidation.

TS and VS show substantial variability among samples, with TS ranging from 34 to 80 g/L and VS from 25 to 60 g/L. Higher values in E3 correspond to recent discharges, while reductions in E1 and E2 suggest mineralization and organic matter degradation during prolonged ponding. Fat concentrations vary between 0.3 and 0.8 g/L, with the highest in E3. The accumulation of fats on the pond surface forms thin hydrophobic films that hinder oxygen transfer and microbial activity, thereby delaying natural biodegradation [3].

Phenolic compounds vary between 5 and 14 g/L, which aligns with the typical range (5–24 g/L) of raw OMW [5,6]. Their high concentrations, particularly in E3, indicate the persistence of refractory organics and limited biodegradation. Phenolic attenuation in older samples (E1 and E2) is likely due to oxidation and adsorption onto sediments. These findings confirm the relative stability of phenolic compounds under natural evaporation conditions.

Major cations (K⁺, Na⁺, Ca²⁺, Mg²⁺) and nutrients (N and P) show moderate variability. Potassium (5.2–7.2 g/L) and sodium (0.12–0.70 g/L) concentrations remain within the ranges reported in previous studies [4,5]. Total nitrogen (1.5–6.8 g/L) and total phosphorus (0.09–1.1 g/L) are enriched, confirming nutrient accumulation within the ponded effluent. The C/N ratios (5.5–12.5) reflect conditions that are suboptimal for biodegradation and humification, favoring the persistence of organic matter [5].

Overall, the Ben Aoun OMW exhibits the typical characteristics of highly concentrated olive mill effluents. These include acidic pH, high COD, and elevated phenolic content. Consequently, the effluent represents a potential long-term source of soil and groundwater contamination.

4.2. Soil Contamination

Lithological profiles from boreholes I, II, and III (Figure 5, Figure 6 and Figure 7) reveal clear evidence of subsurface contamination caused by OMW infiltration. The control site, located approximately 1 km from the disposal area, displays baseline soil characteristics with basic pH (8.5–8.7), low EC (≈0.8 mS/cm), and minimal organic matter (0.13–0.15%).

In contrast, soils within and adjacent to the OMW pond exhibit pronounced vertical variations. In Borehole III, located inside the pond, organic matter content reaches 26% in the upper meter and rapidly decreases to <1% below 1.6 m. Water content follows a similar trend, decreasing from 48% near the surface to 7% at 1.8 m. The pH increases with depth, shifting from acidic near the surface to neutral–alkaline below 1.2 m, inversely correlated with EC, which peaks at 15.5 mS/cm in the upper 40 cm and decreases progressively downward. These variations indicate substantial infiltration of acidic, saline wastewater that enhances cation exchange, promotes the adsorption of phenolic compounds, and drives ion accumulation within clay-rich horizons. Secondary EC peaks at depths of 1.8 m and 4.0 m correspond to organic-rich sandy–clay interlayers, suggesting downward migration of OMW constituents.

Boreholes I and II, located outside the pond, display moderate EC increases (2–4 mS/cm) and slight pH decreases (7.5–6.5) at depths of 1–3 m, consistent with limited lateral infiltration and partial attenuation by clay horizons.

At the control point, soil properties remain stable between 20 and 40 cm depths, with water content ranging from 15 to 16%, organic matter 0.13–0.15%, pH 8.5–8.7, EC 0.75–0.8 mS/cm, and polyphenol content 0.13–0.19 mg Eq AG/g MS.

The geochemical profiles (Figure 8, Figure 9 and Figure 10) confirm these findings. Borehole III exhibits the highest contamination intensity, with acidic pH and high EC in the upper layers, while secondary EC peaks at 1.8–4.2 m correspond to dark, organic-enriched sandy–clay layers. Boreholes I and II show smaller anomalies associated with minor infiltration along clay–sand contacts.

Polyphenol concentrations (

Table 3. Polyphenol content in soil samples collected from the control point and boreholes (I, II, III). Values represent single measurements at each depth or location; no replicates were obtained. This table has been modified to clarify the sampling strategy and to explicitly indicate that each entry corresponds to a single measurement.

Control Point (CP) (mg Eq AG/gMS)	Borehole (I) (mg Eq AG/gMS)	Borehole (II) (mg Eq AG/gMS)	Borehole (III) (mg Eq AG/gMS)
CP20 = 0.13	I80 = 0.15	II60 = 0.19	III40 = 0.34
CP40 = 0.19	I280 = 0.31	II140 = 0.24	III100 = 0.8
-	I440 = 0.36	II280 = 0.26	III120 = 0.3
-	-	II340 = 0.36	III180 = 0.28
-	-	-	III340 = 0.27
-	-	-	III400 = 0.31
-	-	-	III500 = 0.3

) are substantially higher in pond soils than in the control samples, reaching 0.8 mg Eq AG/g MS and remaining elevated down to 5 m depth. The strong adsorption of organic matter and phenolic compounds in clay-rich horizons supports the role of fine-grained materials as temporary sinks for OMW-derived pollutants.

Although the relatively low hydraulic conductivity of these strata limits lateral migration, long-term exposure to acidic effluents promotes carbonate dissolution, structural alteration, and aggregate formation [21,28]. These processes enhance porosity and permeability, potentially facilitating deeper infiltration over time. The persistence of polyphenolic compounds is further sustained by their affinity for clay minerals and their resistance to biodegradation [4].

4.3. Groundwater Quality

Groundwater analyses from three wells surrounding the Ben Aoun pond (

Table 4. Results of physicochemical analysis of water samples.

Samples	1	2	3
Polyphenols (mg/L)	13.17	10	41
pH	7.8	7.3	7.5
EC (mS/cm)	1.9	1.6	2

; Figure 3) demonstrate variable contamination levels. Polyphenol concentrations range between 10 and 41 mg/L, with the highest value observed in well W3, located closest to the pond. Wells situated further away (W1–W2) show lower concentrations (≤14 mg/L). EC varies from 1.6 to 2.0 mS/cm, while pH values remain neutral (7.3–7.8).

Measured polyphenol concentrations exceeded the WHO-recommended limit for drinking water (≤0.001 mg/L), while EC values in some wells also approached or exceeded Tunisian irrigation standards (≤2.5 mS/cm). This combined evidence indicates significant contamination of the shallow aquifer system. The spatial decrease in phenolic concentrations with distance from the pond suggests attenuation by dilution, sorption, and partial biodegradation. However, the detection of phenolics at distances of up to 300 m highlights the persistence and mobility of these compounds. Similar contamination patterns have been documented in other Tunisian OMW disposal sites [4,12].

4.4. Environmental Implications

The integrated analysis of OMW, soil, and groundwater data demonstrates that unlined evaporation ponds cause substantial geochemical alteration of the vadose zone. Infiltrating effluents induce acidification, salinization, and organic enrichment, which modify soil mineralogy through adsorption, ion exchange, and carbonate dissolution. These reactions increase porosity and permeability, thereby enhancing contaminant migration.

Phenolic compounds, particularly monomers, exhibit high persistence due to their low biodegradability and strong affinity for clay minerals. Their retention in surface horizons leads to long-term contamination potential, especially under episodic rainfall and fluctuating water-table conditions.

Despite partial attenuation within clay-rich layers, sustained infiltration and concentration effects from repeated discharges contribute to progressive degradation of soil quality and contamination of groundwater. These findings align with those reported by [4,21], confirming that OMW infiltration alters soil structure, mineralogy, and geochemical equilibrium.

The observed accumulation of polyphenols up to 0.8 mg Eq AG/g MS in pond soils, coupled with groundwater concentrations up to 41 mg/L, underscores the environmental risk posed by uncontrolled OMW storage. The Ben Aoun site thus exemplifies the long-term hazards of non-engineered disposal systems under semi-arid Mediterranean conditions.

The results of this study are consistent with earlier investigations in the Mediterranean Basin, where uncontrolled OMW disposal has caused soil and groundwater contamination. Mekki et al. [4] reported similar enrichment of phenolic compounds and salinity in soils near infiltration basins in central Tunisia, while Paredes et al. [29] observed strong adsorption of phenolic and organic constituents on clayey soils, which limited vertical migration but maintained high surface contamination.

In comparison, the Ben Aoun site exhibits greater vertical migration (up to 5 m) and measurable groundwater contamination, likely reflecting local lithological and hydrogeological conditions that favor infiltration. Similar spatial variability has also been reported across North Africa and southern Europe, where pollutant behavior is influenced by soil texture, permeability, and climatic conditions.

Overall, the high concentrations of COD, EC, and polyphenols exceed WHO and Tunisian guideline limits. This highlights the serious environmental risks associated with uncontrolled OMW disposal. Without appropriate containment or treatment, prolonged use of open ponds can result in soil salinization, loss of fertility, and irreversible deterioration of groundwater quality [30,31,32]. These findings emphasize the urgent need for regulatory control using monitoring boreholes and/or geophysical techniques, improved waste management practices, and the implementation of engineered treatment systems to mitigate the environmental impact of OMW in Tunisia and other Mediterranean regions.

This study supports the achievement of Sustainable Development Goal 6 (Clean Water and Sanitation) by providing critical evidence of groundwater contamination from untreated OMW. The findings highlight the urgent need for improved wastewater management and pollution control measures to protect vital water resources, thereby contributing directly to Target 6.3, which aims to improve water quality by reducing the discharge of hazardous substances.

Geophysical techniques such as electrical resistivity tomography, transient electromagnetic, and radiomagnetotelluric methods offer valuable, non-invasive tools for monitoring subsurface variations in EC [33,34,35] associated with contaminant migration from OMW ponds. Such methods are recommended for these studies, as they can detect spatial and temporal changes in resistivity that reflect salinization, leachate infiltration, and the extent of groundwater contamination. By using time-lapse geophysical surveys, it is possible to track the evolution of pollution plumes, identify preferential flow paths, and evaluate the effectiveness of remediation or containment measures. It should be noted that this study is based on single-point measurements. Nevertheless, the robustness of the interpretations arises from the magnitude of observed contrasts between contaminated and control sites, the consistency of vertical and spatial trends across multiple profiles, and the comparison with established guideline values and literature data. As such, the findings provide reliable field-based evidence of contaminant migration processes under long-term disposal conditions.

5. Conclusions

This study provides an integrated assessment of the environmental impact of OMW disposal at the Ben Aoun evaporation pond in central Tunisia, combining field sampling and laboratory analyses of wastewater, soil, and groundwater. The results demonstrate that uncontrolled OMW storage in unlined basins constitutes a significant source of local contamination, especially in areas characterized by high soil permeability and shallow groundwater.

Field evidence indicates that OMW infiltration extends laterally up to approximately 20 m from the pond margins and vertically to depths of at least 5 m, affecting the upper unsaturated and shallow saturated zones. The acidic and saline nature of OMW promotes geochemical alterations in the subsurface, including partial neutralization of alkaline clay minerals, elevated EC, and accumulation of organic matter and phenolic compounds in the soil. The persistence and mobility of phenolic compounds suggest potential migration through the vadose zone, posing a long-term threat to groundwater quality.

6. Recommendations

To mitigate these impacts, immediate engineering and management interventions are recommended:

• Containment and isolation of the pond through the installation of a low-permeability bottom liner and a perimeter slurry trench to prevent further seepage;
• Continuous environmental monitoring of soil and groundwater quality to detect and track contaminant migration;
• Implementation of pretreatment and valorization options, such as anaerobic digestion, evaporation-concentration, or composting, to reduce pollutant loads before discharge;
• Progressive replacement of open ponds with engineered evaporation basins or integrated treatment systems that combine physical, chemical, and biological processes.

In the broader context, this case study underscores the urgent need for national regulation and technical standards governing OMW management in Tunisia and similar Mediterranean regions. Long-term environmental monitoring and adoption of sustainable waste recovery strategies are essential to minimize the ecological footprint of olive oil production and to ensure the protection of vulnerable groundwater resources.