WET/DRY SAMPLING DURING WATERSHED ASSESSMENT: A STUDY IN THE PIEDMONT OF GEORGIA TO DETERMINE WHAT IS DRY AND WET

In Georgia a wet sampling event is defined as 0.1 or more inches of rain 72 hours before sample collection. Dry events have less than 0.1 inches of rain 72 hours before sampling. Our research focus is on refining what really is wet and dry. Water quality parameters of surface streams vary with rainfall. A single rainfall event might by dilution improve some water quality parameters (DO, pH, specific conductivity) and by flushing in sediment, roadway pollution, and manure degrade the water quality’s turbidity, TSS, fecal coliform bacteria, metals and nutrients. In order to fully understand surface water quality watershed assessments are conducted so that sampling includes both wet and dry events.

We have sampled at 12-hour intervals before, during and after rainfall on three different size streams (small Campus Branch on SUWG campus, Little Tallapoosa River, and Chattahoochee River in Carroll County). Results show that significant rainfall events affect water quality (elevated turbidity, DO, fecal coliform bacteria and lowered specific conductivity) much longer than 72 hours and that lesser rain events may not affect water quality parameters for 72 hours. Further results indicate that the size of the stream is an important factor, with smaller streams strongly affected by 0.1 to 0.2 inches whereas this amount of rain may not change water quality parameters of larger streams. Smaller streams can return to background levels much sooner than larger streams. For example, even 7 days after a significant rain (>1 inch) the turbidity of the Little Tallapoosa River does not return to background levels. During the wet winter months lower temperature, less sunlight, and less water uptake by plants cause the frequent rain events to be very common. If winter dry events are defined as a return of water quality parameters to background levels then dry sampling may be uncommon during the winter. The inverse can occur during prolonged dry periods during the summer with rain events between 0.1 and 0.2 inches having no effect on water quality of surface streams in relatively pristine watersheds. A further complexity is the nature of the watershed. Campus Branch has a high level of impervious surface with the classic rapid high flow response to rain events. It truly appears that one size does not fit all when it comes to defining wet and dry sampling events.