Using vadose zone data to determine agricultural impact on groundwater pollution

Nitrate pollution of groundwater is often attributed to excess fertilisation in agriculture. Low quality water, enriched by nitrate and other pollutants percolates down from the root zone through the unsaturated zone to the water table. Accordingly, the vadose zone holds the footprint of all possible groundwater pollution events occurring at the land surface before the pollution imprint arrives in the groundwater. Here we present a study were detailed long-term monitoring of the unsaturated zone reveals the groundwater pollution potential of various representative agricultural setups over the island of Malta. Malta is a semi-arid island in the Mediterranean Sea where groundwater, which is the only natural freshwater resource, suffers from nitrate pollution due to the intensive agricultural landscape. A national monitoring network, comprising of 16 Vadose zone Monitoring Systems (VMS) were installed under the different agricultural setups which represent Malta’s main agricultural practices. The VMS enables continuous monitoring of variations in the unsaturated zone water content, as indication to percolation processes, and frequent sampling of the sediment pore water for chemical analysis and characterisation of pollutant transport across the unsaturated zone.  

Results show that the mean nitrate concentrations in the vadose zone underlying fields of potato, forage, mixed outdoor vegetables, greenhouses, vineyards, and orchards were 923 mg/L, 673 mg/L, 416 mg/L, 416 mg/L, 252 mg/L and 33.6 mg/L, respectively. Spatial distribution of the different agricultural setups shows that forage and potato fields are among the most common agricultural setups, with higher occurrence in the central and eastern areas of the island (>50% of the agricultural land area). This agricultural land use distribution spatially links to the high average nitrate concentrations in groundwater under those fields (ranging from 75 to 200 mg/L). On the other hand, lower proportions of potato and wheat fields are cultivated in the north-western areas (25-50% of the total agricultural land area). In the north-western areas, a clay layer situated in the unsaturated zone creates a shallow perched aquifer with a rock matric thickness ranging from 20 to 50 m, which impedes water fluxes from the agricultural fields to the main groundwater system below. The mean nitrate concentrations in the shallow perched aquifer are relatively high ranging from 200 to 350 mg/L due to the aquifer’s low water storage. On the other hand, nitrate concentrations in the regional aquifer underlying the perched aquifer are relatively low ranging from 25 to 100 mg/L.

In conclusion, the results show that potato and wheat fields are likely to have the greatest impact on nitrate pollution in the vadose zone and eventual groundwater nitrate contamination. Furthermore, these agricultural land uses are among the most common land uses cultivated in Malta. This implies the significant potential spatial impact of potato and wheat fields on groundwater nitrate pollution. With data being made available from this vadose zone monitoring network we can increasingly understand the pollution potential of different agricultural land uses on groundwater.