Inputs of Total and Labile Dissolved Metals from Six Facilities Continuously Discharging Treated Wastewaters to the Marine Environment of Gran Canaria Island (Canary Islands, Spain)
2. Materials and Methods
2.1. Sampling Facilities
- TP-1: WWTP, which mainly collects household wastewater from the city of Las Palmas de Gran Canaria. This plant was considered as a reference for a typical large Canarian WWTP with domestic inputs.
- TP-2 and TP-3: medium size WWTP, which collects mixed household and industrial sewage inlets.
- TP-4: WWTP, which collects only industrial wastewater from two industrial areas of the island.
- TP-5: a coastal thermal power plant, which produces one of the largest discharges of cooling wastewater in the Canary Islands.
- TP-6: an indoor seawater aquaculture (fish) farm.
2.2. Sample Collection
- In situ parameters measurements
2.3. Sample Analyses
- Trace elements in DGTs by ICP-MS
- Trace elements in spot water samples by ICP-MS
2.4. Data Processing
- Treatment of DGT-labile-fraction metal concentration data
- Calculation of the mass of metal (M), in g units, accumulated in the resin-gel layer, according to Equation (1):M = Ce ∗ (VHNO3 + Vgel)/fe
- Ce is the concentration of metals, in g·L−1 units, in the 1 M HNO3 elution solution
- VHNO3 is the volume of HNO3 added to the resin gel (1.25 mL in this study)
- Vgel is the volume of the resin gel (typically 0.15 mL)
- fe is the elution factor for each metal (typically 0.8)
- Calculation of the concentration of metal in water, in g·L−1 units, measured by the DGT device (CDGT), according to Equation (2):CDGT = (M ∗ Δg)/(D ∗ t ∗ A)
- Δg is the thickness, in cm units, of the diffusive gel (approx. 0.08 cm) plus the thickness of the filter membrane (0.014 cm)
- D is the diffusion coefficient of metal in the gel, available at :
- t is deployment time (in s units)
- A is the exposure area (3.14 cm2)
- Treatment of Spot-sampling dissolved metal concentration data
- Verify that the mean concentrations of each metal in the exposed DGTs at each sampling site were higher than those in the DGTs blanks at the laboratory.
- Verify that, in the TP-6 results, the labile-fraction metal concentration (based on DGTs results, in triplicate) was higher than the total dissolved metal concentration (based on spot sampling results, in triplicate).
3. Results and Discussion
3.1. DGT Blanks
3.2. Concentrations of Total Dissolved and Dissolved Labile Metals
3.2.1. Total Dissolved Metals
3.2.2. Dissolved Labile Metals
3.2.3. Comparison between the Concentrations of the Total-Dissolved and the Labile Metal Fractions
- Differences in the fraction measured in spot sampling (total dissolved) and by DGTs (dissolved labile). As mentioned before, different chemical forms are measured depending on the fraction considered. The concentrations found of total dissolved metals tended to be generally higher than the DGT-labile concentrations, so the percentage of the labile fraction being part of the total dissolved fraction is normally less than 100% (Table 2). Although, some exceptions were observed, mainly in the TP-6 facility, where the percentage of the labile fraction per the total dissolved concentration exceeded 100% in most of the studied metals except for Cd, Ni, and Zn (Supplementary Information S3).
- Differences in the timescale of the spot-sampling measurements and the DGTs measurements. Results do not represent the same sampling timescale (Table 2). Total dissolved metal concentrations are the average of the metal concentrations measured at three specific times (day 0, 2, and 4), whereas the DGT provides 4 days-weighted average metal concentrations. Thus, the spot sampling can miss some peaks and/or decreases in metal concentrations and may not properly monitor the wide variation in the total dissolved metal content. The advantage of using DGT devices is their ability to measure time-weighted average concentrations over the deployment period providing more representative results for highly variable systems. This seems to be especially relevant in the case of the marine fish farm facility (TP- 6), where differences in the temporal distribution of the farming processes (feeding, use of chemicals, water recirculation, waste load, etc.) may affect the temporal content and speciation of metals significantly .
- Differences in the physicochemical characteristics of the analyzed effluents. The physicochemical conditions may impact the forms’ distribution (speciation) for a given metal. In this study, we determined the physicochemical parameters in the different wastewater effluents at each sampling day (Supplementary Information S4). The overall temperature in the 6 outlets ranged between 21.9 and 29.0 °C. The highest values in temperature were measured at TP-3 (with domestic and industrial influents), whereas the lowest temperatures were registered at the marine fish farm (TP-6). The pH values ranged between 4.73, measured in the TP-4 effluent (treating industrial influents by flocculation), and 7.73, in TP-3 (with mixed domestic and industrial influents). Furthermore, the lowest dissolved oxygen values were registered at the TP-4 facility (0.36 mg·L−1), while, at the other facilities, the dissolved oxygen ranged between 6.19 and 7.31 mg·L−1. In addition, as some of the sampled effluents’ water source was seawater (TP-5 cooling water and TP-6 marine fish farm), the overall recorded conductivity (25 °C) ranged widely, between 1.66 and 55.67 mS·cm−1.
3.3. Effluent Discharge Impact on Coastal Water Bodies
- ci is the mean concentration (total dissolved or labile) of each metal in the ith sample measured in this study for each facility
- qi is the daily average flow from each facility, and ti is the time interval (1 day).
Institutional Review Board Statement
Informed Consent Statement
Conflicts of Interest
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|Label||Type of Facility 1||Wastewater Source||Size (p.e.) 2||Main Treatment||Discharge Flow (m3/h)|
|TP-1||WWTP||Household||600,000||Pre-treatment/Settling Activated sludge Chlorination||1100|
|TP-2||WWTP||Household + industry (10%)||171,600||Pre-treatment/Settling Activated sludge Chlorination||345|
|TP-3||WWTP||Household + industry (25%)||50,000||Pre-treatment Membrane bioreactor (MBR) Chlorination||136|
|TP-4||WWTP||Industry||4600||Pre-treatment Coagulation–flocculation settling||30|
|TP-5||TPP||Industry (cooling water of a thermal power plant)||No data||Aeration tanks/Settling||27,500|
|TP-6||MFF||Aquaculture (indoor marine fish-farm)||No data||Settling||75|
|Cd||Dissolved, day 0||0.002||0.005||0.009||0.090||0.010||0.010|
|Dissolved, day 2||0.002||0.010||0.013||0.088||0.017||0.012|
|Dissolved, day 4||0.001||0.004||0.013||0.017||0.013||0.010|
|Dissolved (mean ± SD)||0.002 ± 0||0.007 ± 0.003||0.012 ± 0.002||0.065 ± 0.041||0.013 ± 0.004||0.011 ± 0.001|
|Ni||Dissolved, day 0||2.242||2.820||1.104||18.319||3.700||0.146|
|Dissolved, day 2||3.665||4.304||1.157||21.829||2.707||0.117|
|Dissolved, day 4||3.939||2.904||1.169||13.984||9.347||0.069|
|Dissolved (mean ± SD)||3.282 ± 0.911||3.343 ± 0.833||1.143 ± 0.035||18.044 ± 3.930||5.251 ± 3.581||0.011 ± 0.001|
|Pb||Dissolved, day 0||0.350||0.219||0.205||0.901||0.009||0.009|
|Dissolved, day 2||0.377||0.353||0.130||0.764||0.067||0.009|
|Dissolved, day 4||0.387||0.254||0.157||0.076||0.009||0.009|
|Dissolved (mean ± SD)||0.371 ± 0.019||0.276 ± 0.069||0.164 ± 0.038||0.580 ± 0.442||0.028 ± 0.033||0.009 ± 0|
|Cr||Dissolved, day 0||0.786||0.447||0.500||4.296||0.133||0.196|
|Dissolved, day 2||1.207||0.582||0.503||4.730||0.170||0.190|
|Dissolved, day 4||1.312||0.360||0.498||1.612||0.212||0.167|
|Dissolved (mean ± SD)||1.102 ± 0.278||0.463 ± 0.112||0.5 ± 0.003||3.546 ± 1.689||0.172 ± 0.040||0.185 ± 0.015|
|Cu||Dissolved, day 0||0.242||0.438||2.300||2.385||0.015||0.015|
|Dissolved, day 2||0.135||1.330||1.666||1.998||0.015||0.015|
|Dissolved, day 4||0.182||0.403||1.356||0.440||0.015||0.015|
|Dissolved (mean ± SD)||0.186 ± 0.054||0.724 ± 0.526||1.774 ± 0.481||1.608 ± 1.03||0.015 ± 0||0.015 ± 0|
|Zn||Dissolved, day 0||5.328||19.201||30.643||125.712||20.115||6.518|
|Dissolved, day 2||5.249||26.584||31.016||136.248||21.053||6.413|
|Dissolved, day 4||4.903||20.147||32.818||82.488||5.059||5.294|
|Dissolved (mean ± SD)||5.160 ± 0.226||21.977 ± 4.017||31.492 ± 1.163||114.816 ± 28.488||15.409 ± 8.976||6.075 ± 0.679|
|Al||Dissolved, day 0||29.823||72.254||17.724||911.029||0.861||0.861|
|Dissolved, day 2||32.419||44.602||20.245||1056.066||0.861||0.748|
|Dissolved, day 4||32.103||95.000||21.680||124.072||6.898||0.748|
|Dissolved (mean ± SD)||31.448 ± 1.417||70.619 ± 25.238||19.883 ± 2.003||697.056 ± 501.489||2.874 ± 3.486||0.786 ± 0.065|
|Fe||Dissolved, day 0||101.001||58.643||59.049||13,048.876||0.207||0.482|
|Dissolved, day 2||114.755||108.238||52.918||11,627.790||0.207||0.207|
|Dissolved, day 4||118.142||73.133||56.386||4890.290||1.118||0.207|
|Dissolved (mean ± SD)||111.299 ± 9.078||80.005 ± 25.501||56.118 ± 3.074||9855.652 ± 4358.438||0.511 ± 0.526||0.299 ± 0.159|
|Mn||Dissolved, day 0||30.919||14.048||1.328||829.592||1.335||1.234|
|Dissolved, day 2||32.832||62.795||0.223||718.726||1.432||1.283|
|Dissolved, day 4||35.490||43.483||3.285||381.830||1.971||1.116|
|Dissolved (mean ± SD)||33.080 ± 2.295||40.109 ± 24.548||1.612 ± 1.550||643.383 ± 233.195||1.579 ± 0.342||1.211 ± 0.086|
|Co||Dissolved, day 0||0.251||0.613||0.283||7.344||0.360||0.007|
|Dissolved, day 2||0.649||0.866||0.299||8.488||0.247||0.006|
|Dissolved, day 4||0.646||0.544||0.321||4.146||1.049||0.008|
|Dissolved (mean ± SD)||0.515 ± 0.229||0.674 ± 0.17||0.301 ± 0.019||6.659 ± 2.251||0.552 ± 0.434||0.007 ± 0.001|
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Rodrigo Sanz, M.; Millán Gabet, V.; Gonzalez, J.-L. Inputs of Total and Labile Dissolved Metals from Six Facilities Continuously Discharging Treated Wastewaters to the Marine Environment of Gran Canaria Island (Canary Islands, Spain). Int. J. Environ. Res. Public Health 2021, 18, 11582. https://doi.org/10.3390/ijerph182111582
Rodrigo Sanz M, Millán Gabet V, Gonzalez J-L. Inputs of Total and Labile Dissolved Metals from Six Facilities Continuously Discharging Treated Wastewaters to the Marine Environment of Gran Canaria Island (Canary Islands, Spain). International Journal of Environmental Research and Public Health. 2021; 18(21):11582. https://doi.org/10.3390/ijerph182111582Chicago/Turabian Style
Rodrigo Sanz, Marta, Vanessa Millán Gabet, and Jean-Louis Gonzalez. 2021. "Inputs of Total and Labile Dissolved Metals from Six Facilities Continuously Discharging Treated Wastewaters to the Marine Environment of Gran Canaria Island (Canary Islands, Spain)" International Journal of Environmental Research and Public Health 18, no. 21: 11582. https://doi.org/10.3390/ijerph182111582