Assessment of Intrinsic Vulnerability Using DRASTIC vs. Actual Nitrate Pollution: The Case of a Detrital Aquifer Impacted by Intensive Agriculture in Cádiz (Southern Spain)
Abstract
:1. Introduction
2. Study Area
3. Methodology
3.1. DRASTIC Model
- The depth of the water table (D) was calculated in each cell by subtracting the piezometric level from ground elevation. Ground elevation was obtained from the official digital terrain model known as MDT05 (Modelo Digital del Terreno 5 m × 5 m), produced by the Spanish National Cartographic Institute (IGN), whereas the piezometric level was inferred from an isopiestic map previously made from water table observations gathered by the authors in May 2019.
- Recharge (R) was obtained from the distribution map of average annual precipitation in the area, which was generated using the SIMPA model (Integrated System for Precipitation-Contribution Modelling) for the period 1940–2005 [33] by considering an average recharge of 27% precipitation. This percentage would be considered typical for wet years and would therefore represent the most unfavourable situation from the point of view of contamination.
- The nature of the aquifer (Aquifer Media, A) was defined according to the outcrop lithology described in the official geological cartography produced by the Spanish Geological Survey (IGME) at a scale of 1:50,000.
- The soil parameter (S) was obtained from the Soils Map of Andalusia at a scale of 1:400,000, which was produced by the Ministry of the Environment of the Andalusian Regional Government in 2005 [33].
- The slope parameter (Topography, T) was obtained from the slope map generated from the MDT05 [34].
- The Impact of the Vadose Zone (I) was the most difficult variable to estimate owing to the limited availability of lithological columns from boreholes in the aquifer. For this reason, and given the subhorizontal disposition and nature of the layers, it was decided to assign an average value for the entire area. Accordingly, an unfavourable scenario with the presence of highly permeable material (calcarenites and sandstones) in the vadose zone was considered.
- Lastly, the hydraulic conductivity was calculated from 4 pumping tests conducted on the different materials identified in the aquifer: Upper Miocene calcarenites, Pliocene sands, and aeolian mantle. The results of the pumping tests were interpreted using the PIBE 3.2 software [35].
3.2. Groundwater Sampling
3.3. Multispectral Satellite Imagery to Identify Irrigated Plots
4. Results and Discussion
4.1. DRASTIC Vulnerability Maps
4.2. Concentration and Spatial Distribution of Contaminants
4.3. Comparison between the Vulnerability Index and Actual Contamination
4.4. Correlation between Nitrate Concentration and Other Variables
4.5. Influence of the Type of Sampling Point and Sampling Procedure on Nitrate Content
4.6. Denitrification Processes in the Porous Media
5. Conclusions
- 1.
- Land use is markedly heterogeneous. Therefore, the spatial distribution of nitrogen sources shows great spatial variability. In this regard, it should be noted that the distribution of land uses considered in the official documents only corresponds partially to the actual uses and that the spatial distribution of nitrate cannot be explained solely by the proximity of the sampling points to the sources of contamination, not even when considering the advective transport process inferred from the piezometry.
- 2.
- The sampling procedure significantly conditions the analytical results of nitrate concentration. In the porous media, the ion NO3− displays vertical stratification; those boreholes that are deeper and subject to larger pumping rates present lower values of this contaminant, whereas the shallower boreholes with low-power pumps, or those points that have been sampled manually, present higher concentrations. In this respect, the design of an adequate control network is crucial to accurately picture the extent of the contamination within the aquifer. The use of isolated control points or a sparse control network may not be representative of the whole aquifer.
- 3.
- A significant relationship was identified between the concentration of nitrates and water salinity as well as with other conservative ions (chloride and bromide). This fact is explained by the salt accumulation process caused by the recirculation of the groundwater pumped and applied over the irrigation plots located on the aquifer surface, which under a Mediterranean climate with high evaporation rates, worsen the salt concentration problem.
- 4.
- At some sampling points there is evidence of reducing environments which favour higher concentrations of Fe and Mn and lower values of nitrate and ORP. In this type of environment, the degradation of NO3− takes place with varying intensities. In particular, in the study area, there is a sampling point where nitrate degradation is of special importance, which is attributable to denitrification processes by anaerobic bacteria linked to anoxic levels with abundant organic matter.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
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D | Depth to water (m) | Range | <1.5 | 1.5–5 | 5–10 | 10–20 | 20–30 | >30 |
Rating | 10 | 9 | 7 | 5 | 2 | 1 | ||
R | Recharge (mm) | Range | 180–225 | >225 | ||||
Rating | 8 | 9 | ||||||
A | Aquifer | Description | Clays, Sands and Gravels (Quaternary) | Yellow Sands (Pliocene) | Aeolian mantle Sands (Quaternary) | Calcarenites (Miocene) | ||
Range | 3–5 | 4–9 | 5–9 | 9–10 | ||||
Rating | 4 | 7 | 8 | 9 | ||||
S | Soil | Soil type | Calcic and Chromic Luvisols, Calcareous Cambisols | Regosols and Cambisols, Limestones | Chromic Vertisols and Vertic Cambisols | |||
Rating | 1 | 6 | 7 | |||||
T | Topography (%) | Range | 0–2 | 2–6 | 6–12 | 12–18 | >18 | |
Rating | 10 | 9 | 5 | 3 | 1 | |||
I | Impact of the vadose zone | Description | Silts, Clays, Shales | Calcarenites, Sandstones, Limestones | Sands and Gravels | |||
Range | 1–6 | 2–7 | 6–9 | |||||
Rating | 6 | 6 | 6 | |||||
C | Hydraulic Conductivity (m/day) | Material | Clays, Sands and Gravels (Quaternary) | Yellow Sands (Pliocene) | Aeolian mantle Sands (Quaternary) | Calcarenites (Miocene) | ||
Range | <4 | 4–12 | 12–28 | 28–80 | ||||
Rating | 1 | 2 | 2 | 8 |
Parameter | Weight Coefficient |
---|---|
Depth to water (D) | 5 |
Recharge (R) | 4 |
Aquifer media (A) | 3 |
Soil media (S) | 2 |
Topography (T) | 1 |
Impact of the vadose zone (I) | 5 |
Hydraulic conductivity (C) | 3 |
VI Value | Vulnerability |
---|---|
<79 | Minimum |
80–99 | Very Low |
100–119 | Low |
120–139 | Medium–Low |
140–159 | Medium–High |
160–179 | High |
180–199 | Very high |
>200 | Maximum |
Cod | Type | Water Table Depth (m) | Pump Depth (m) | Annual Volume Pumped (m3/year × 1000) | Average Discharge Flow (L/s) |
---|---|---|---|---|---|
1 | Uninstalled well | 10.6 | - | 0 | |
2 | Installed well | 17.8 | ND | ND | |
3 | Installed well | 23.3 | 63 | 150 | |
4 | Installed well | 23.3 | 28 | 1 | |
5 | Spring | 0.0 | 15 | ||
6 | Installed well | 4.4 | 56 | 120 | |
7 | Installed well | 22.6 | ND | ND | |
8 | Installed well | 10.7 | 42 | 6 | |
9 | Installed well | 24.2 | 54 | 15 | |
10 | Spring | 0.0 | 0.2 | ||
11 | Installed well | 36.7 | 70 | 81 | |
12 | Installed well | 35.4 | 110 | 430 | |
13 | Installed well | 24.0 | 39 | 10 | |
14 | Large diameter well | 7.8 | - | 0 | |
15 | Installed well | 9.3 | ND | ND | |
16 | Spring | 0.0 | 4 | ||
17 | Piezometer | 5.8 | - | 0 | |
18 | Large diameter well | 3.0 | - | 0 | |
19 | Installed well | 1.7 | 30 | 180 | |
20 | Installed well | 2.4 | ND | ND |
Index Value | Vulnerability Class | Area (km2) | Area (%) |
---|---|---|---|
<120 | Low | 3.4 | 10.4 |
120–139 | Medium–Low | 7.5 | 23.0 |
140–159.9 | Medium–High | 10.5 | 32.2 |
160–179 | High | 4.8 | 14.7 |
>180 | Very High | 6.4 | 19.6 |
Total | 32.6 | 100 |
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Chilaule, S.M.; Vélez-Nicolás, M.; Ruiz-Ortiz, V.; Sánchez-Bellón, Á.; García-López, S. Assessment of Intrinsic Vulnerability Using DRASTIC vs. Actual Nitrate Pollution: The Case of a Detrital Aquifer Impacted by Intensive Agriculture in Cádiz (Southern Spain). Agriculture 2023, 13, 1082. https://doi.org/10.3390/agriculture13051082
Chilaule SM, Vélez-Nicolás M, Ruiz-Ortiz V, Sánchez-Bellón Á, García-López S. Assessment of Intrinsic Vulnerability Using DRASTIC vs. Actual Nitrate Pollution: The Case of a Detrital Aquifer Impacted by Intensive Agriculture in Cádiz (Southern Spain). Agriculture. 2023; 13(5):1082. https://doi.org/10.3390/agriculture13051082
Chicago/Turabian StyleChilaule, Sérgio Mateus, Mercedes Vélez-Nicolás, Verónica Ruiz-Ortiz, Ángel Sánchez-Bellón, and Santiago García-López. 2023. "Assessment of Intrinsic Vulnerability Using DRASTIC vs. Actual Nitrate Pollution: The Case of a Detrital Aquifer Impacted by Intensive Agriculture in Cádiz (Southern Spain)" Agriculture 13, no. 5: 1082. https://doi.org/10.3390/agriculture13051082
APA StyleChilaule, S. M., Vélez-Nicolás, M., Ruiz-Ortiz, V., Sánchez-Bellón, Á., & García-López, S. (2023). Assessment of Intrinsic Vulnerability Using DRASTIC vs. Actual Nitrate Pollution: The Case of a Detrital Aquifer Impacted by Intensive Agriculture in Cádiz (Southern Spain). Agriculture, 13(5), 1082. https://doi.org/10.3390/agriculture13051082