Nitrate concentration changes in stream of Lithuanian karst zone within the context groundwater-surface water interaction

Abstract

The quality of groundwater was observed in three wells. The well 1 (depth 6 m) was arranged in a sandy loam soil, in a drained area at a distance of 1 m from the streambed; the well 2 (depth 4 m) was arranged in a peat soil, between the stream G-l and a sinkhole. The sinkhole covered with peat is approximately 10-15 m in diameter, over grown with reeds, often waterlogged by surface water. The well 3 (depth 1.5 m) was arrange in this sinkhole. When stream catchments was drained, water quality and discharge was measured in the drainage system (area 4 ha) the outlet of which is at 10 m distance from the well 1. The area of the drainage system is over grown with perennial grass. Water quality of the stream was measured nearby the well. Water samples were collected once per month in all study places. Nitrates leached out in drainage influence the small stream water quality. Large amounts of nitrogen are leached through drains; which determines the concentrations of this element contained in stream water. When the ground water level fluctuated at a distance less than 120 cm from the ground surface, the strongest correlation of the change in nitrate concentration in the groundwater and stream water was identified. With the decrease of the ground water level, the change of nitrate concentration in the water was less related to the stream water quality. In the well 3 located in the sinkhole, the nitrate concentration was higher than in the other wells. Eutrophication of freshwater environments represents a major threat to the ecological health of surface water in agricultural catchments. High nutrient concentrations, caused by direct inputs into the surface water or indirect contributions via groundwater pathways, can result in excess algae growth. The transport and transformation of nitrate between groundwater and surface water may influence the success of restoration measures to improve stream and groundwater ecological conditions (Krause et al., 2009). The Lithuanian karst zone has specific geomorphological conditions. Here the sinkholes that formed in the course of karst processes create suitable conditions for surface water and pollutants to get into the subsurface water. The bigger rivers have more intensive mixing processes in the flow and receive runoff from greater areas, while quality of stream is under immediate impact of local factors and distinguished impact of particular factors in the catchments (Gaigalis and Smitiene, 2004). This study investigates the change of nitrate concentration in the small stream at the groundwater-surface water interaction. The studies were carried out in moraine sandy loam and peat soils of Lithuanian active karst zone. The site is situated at the headwater of stream G-l (tributary of the Apascia, area 1.63 km2). Active karst processes develop in these gypsum layers. The layers of gypsum-dolomite occur at 5-10 m depth from the soil surface (gypsum layers occur deeper) of the study area.