Présentations Colloques

Session 7.03: Agricultural and sanitation contaminants and implications for water services and health
Gorski Jozef
Influence of groundwater on surface water pollution with nitrates
Groundwater is often polluted by nitrates. This not only causes degradation of the water resources but also affects pollution of surface water which is largely fed by groundwater discharge. In order to evaluate this phenomenon and its changes over a longer period of time the variability of nitrate concentration in the water of the Warta river in the last 65 years was analyzed. Investigations were carried out on the Poznan gauging section closing catchment area of 25083 km2. In the catchment direct groundwater discharge to the river represents 65.5% of the whole river outflow. It was found a noticeably increasing trend in the maximum concentrations of nitrates marked in the winter periods from approx. 2 mgNNO3 L to more than 8 mgNNO3 L in recent decades. This trend is correlated with the agricultural use of nitrogenous fertilizers in Poland. It shows that nitrates in surface water comes mainly from agricultural lands through groundwater. It was also found that graph of nitrate concentration exhibits a distinct maxima (up to 18 mgNNO3 L), which can be correlated with the occurrence of prolonged periods of droughts. During the droughts nitrates loads are accumulated in the soil and unsaturated zone, and then after the drought accumulated nitrates recharge groundwater and further flow into surface waters. The phenomenon of nitrates accumulation during the four-year drought in 1989-1992 was also demonstrated on the basis of nitrates tests in groundwater in several wells and observation wells in the valley of the Warta river. The results demonstrate the usefulness of nitrate concentrations research in surface waters in order to assess the state of groundwater pollution. They also have a high educational value showing the relationship between groundwater and surface water, as well as the need for effective protection of groundwater in order to protect surface waters quality.