EFFECT OF RAINFALL ON WATER QUALITY PARAMETERS: A STUDY OF CORRELATIONS BETWEEN RAINFALL AND WATER QUALITY PARAMETERS

Rainfall can be a significant source of variation in surface-water quality. Runoff can improve, degrade or not alter the water quality of streams depending on the land use, slope, soil type, amount of impervious surfaces, and duration and intensity of the rainfall event. Using a large data set of over 40,000 water quality parameters (West Georgia Watershed Assessment) based on weekly or 6 wet and 6 dry sampling frequencies, correlations can be determined between rainfall and water quality parameters. Some states consider a wet sample event as equal to or greater than 0.1 inch of rain within 72 hours of the sample event. Additional data was collected at two sites at the hydrogeology station at the State University of West Georgia. Beginning in the summer of 2002 sample stations on the Little Tallapoosa River and a small tributary stream were sampled at 12-hour intervals after rain events to more precisely define rainfall’s effect on water quality parameters. Rainfall is expected to have high correlations with turbidity, TSS, fecal coliform bacteria, and nutrients.

Results indicate that in west Georgia, the best correlation of rainfall versus water quality parameters occur when the rain event occurs 24 hours or less before the sampling event. Low rain events (<0.2 inches) that occur between 24 and 72 hours before sampling events in stable second-growth forest watersheds show no correlations between rainfall and water quality parameters. Watersheds in transition from forest and agricultural to suburban sprawl will produce 24 hour-rainfall correlations with turbidity (r²=0.70 for Snake Creek). Streams in areas of livestock grazing (i.e., Yellowdirt Creek) produce 24 hour-rainfall correlations with turbidity (r²=0.66) and nutrients such as phosphorus (r²=0.56).