Modeling Phosphorus Loading to Lake Allatoona: Implications for Water Quality Trading

Lake Allatoona, a large reservoir north of Atlanta Georgia that drains an area of about 2870 km, is threatened by excessive algal growth and scheduled for a phosphorus (P) TMDL. In this paper, we use the Soil Water Assessment Tool (SWAT) computer model to estimate the total P load to Lake Allatoona during the periods 1992-1996 and 2001-2004. We also use the model to estimate the contribution from different sources in the watershed. The total P load to Lake Allatoona increased by 20% between the two time periods. The contribution from point sources decreased from 30% to 13% of the total load due to permit restrictions on P for poultry processing plants. The largest nonpoint source of P was estimated to be forest land use in 1992-1996 accounting for 31% of the load and urban land use in 2001-2004 accounting for 50% of the load. Poultry/cattle land use accounted for 18% in 1992-1996 and 15% in 2001-2004. The implications for a program to trade P credits are: 1) point sources and poultry/cattle operations account for similar percentages of the current load, 2) urban development accounts for most of the current P load and should be brought into a trading program, 3) poultry processing plants that have not upgraded to better P removal technology might trade their current load to wastewater treatment facilities that accept their wastewater, 4) cattle in streams and row crops are not large sources according to our model, and 5) there is little net loss of P to streams during transport to Lake Allatoona so distance of a source from the lake may not be important in a trading scheme.