Testing potential mitigation measures to reduce nutrient loads to a temporary river

To achieve and maintain the good ecological status of surface waters, each EU Member State is expected to invest economic resources in a programme of measures. To do this, a preliminary analysis to quantify the anthropogenic pressures and their impacts on surface waters is necessary to prioritise the measures to be undertaken.

The objectives of this work were to: (i) quantify the nutrient loads from point and diffuse pollution to the Rio Mannu stream identifying the critical time in terms of water quality, and (ii) simulate some mitigation measures for reducing the nutrient loads being delivered to the wetland. Two “measures” were tested to mitigate nutrient pollution in high flow and low flow conditions: (1) the use of treated wastewater from urban wastewater treatment plants (WWTPs) for the irrigation of cultivated olive trees and a (2) reduction in fertiliser usage rates (20%).

Results showed that under high flow conditions, NO₃-N and TP loads accounted for 89% and 99% of the total load, respectively. The low flow contribution to the total load was very low, with NO₃-N and TP accounting for 2.8% and 0.7%, respectively. However, the natural hydrological regime in the study area is intermittent, and low flow represents a critical condition for the water quality due to the high concentrations of TP and NO₃-N from WWTP discharge. Results indicated that agriculture is the main source of NO₃-N in the surface waters. The point sources made a minor contribution, in terms of nutrient load to the surface waters, but they constitute a relevant hydrological pressure for the Rio Mannu, producing a shift from an intermittent hydrological regime (natural conditions) towards a perennial regime. The measures are effective to reduce nutrient loads in surface waters at the outlet for all hydrological conditions. Under high flow conditions, the reduction was 9% and 12% for NO₃-N and TP loads, respectively. The reduction increased under normal and low flow conditions (75% and 83% for NO₃-N and TP loads, respectively).

Based on this study, it is evident that a “programme of measures” for improving the current status of surface waters must be oriented towards reducing nutrient loads, both under high and low flow conditions. The results show that a 20% reduction in fertiliser usage on the main crops in this area, and the reuse of wastewater from selected WWTPs, would result in a significant reduction in the nutrient loads delivered to the wetland, although this reduction is not large enough, and supplementary measures would be required.