Groundwater delivery rate of nitrate and predicted change in nitrate concentration in Blue Lake, South Australia

Abstract

Blue Lake, the principal water supply for the City of Mount Gambier (South Australia), is contaminated with nitrate (NO₃^–) from polluted groundwater. Using existing data, a study was undertaken to determine the past load of NO₃^– from groundwater entering the lake and to forecast future trends in lake NO₃^– concentration. Groundwater NO₃^– loads for the 1971–1997 period were estimated with an inverse model, which combined the long-term record for NO₃^– concentration in the lake with a simple NO₃^– mass-balance. Model results show that the load of NO₃^– from groundwater (18–24 metric tons (t) year^–1 as N) was by far the largest source to Blue Lake between 1971 and 1997. Sinks for NO₃^– included pumping withdrawal (10–14 t year^–1), in-lake consumption (7–10 t year^–1), and groundwater outflow (0–1.8 t year^–1). The NO₃^– concentration in incoming groundwater (4–7 mg N L^–1) appears to have increased slowly but steadily during the 1971–1997 period (at a rate varying between 0.037 and 0.070 mg N L^–1 year^–1). By assuming that the rate of increase in groundwater NO₃^– concentration will remain constant, a forecast for lakewater NO₃^– concentration was made for the 1998–2028 period. Lakewater NO₃^– concentration should increase from the contemporary ~3.5 mg N L^–1 to 4 or 5 mg N L^–1 by 2028. In the short term (decades), the rate of pumping withdrawal will be the main determinant of NO₃^– concentration in the lake through its impact on the rate of groundwater inflow and the lake water residence time. Although the drinking water guideline for NO₃^– (11. 3 mg N L^–1) may not be exceeded in the short term (decades), it may be exceeded in the longer term (centuries) as NO₃^– concentration in the neighbouring aquifer adjusts to the contemporary land use.