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

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.