STORMCHASER: THE 12-HOUR SAMPLE INTERVAL FOR WATER QUALITY ANALYSIS OF SURFACE STREAMS IN THE PIEDMONT AND VALLEY AND RIDGE OF WEST GEORGIA

Quantifying rainfall events is an important part of watershed assessments, though rainfall can be a complex phenomenon. Water quality parameters vary with respect to runoff volume during rain events and dry weather antecedent conditions. Watershed assessments are often designed to capture several wet events. In Georgia, the Environmental Protection Division defines a wet event as one in which 0.1 inches of rain has fallen within 72 hours of sample collection, while a dry event occurs when less than 0.1 inches of rain has fallen within the same time period. Is this definition adequate? To help answer this question, an intense 12-hour water quality sampling interval was conducted at four different streams, three located in the Piedmont Province and one in the Valley and Ridge Province in Georgia. The streams were of varying sizes, were located within 3 different drainage basins (the Tallapoosa, Coosa, and Chattahoochee basins), and contained varying land usages within their watersheds. The Little Tallapoosa River flows through mainly forested and agricultural lands. Euharlee Creek in the Coosa watershed has its headwaters in forested and agricultural lands, but is an urban stream at the sample site location. Small Campus Branch is a tributary of the Little Tallapoosa River that drains the fertilized sports fields of the State University of West Georgia campus. The Chattahoochee River was sampled ~56 miles downstream of Atlanta. In this study, it appears that smaller rain events in forested watersheds have a lesser effect than the same rainfall within an urban watershed and may not affect any change in water quality parameters. When streams can be sampled within 24 hours of a rain event, a much greater correlation between rainfall and water quality parameters results.

In addition to rainfall, other factors need to be considered in wet/dry water quality analysis. Results indicate that in determining wet and dry events, one factor to include is a land usage scale based on percent of impervious cover and forest. Our results indicate that, in areas with abundant impervious cover, sampling within 12 hours of a 0.1 inch rain event does represent a true wet event. Other important factors are stream size and rainfall amount, with a shorter available collection time in smaller watersheds and a longer wait for a dry event in larger watersheds.