Evaluating the Impact of Onsite Wastewater Treatment Systems on Watershed Contamination, Choccolocco Creek Watershed, Alabama

Abstract

There is increasing evidence that onsite wastewater treatment systems (OWTSs) can be a significant, and possibly underestimated, source of water contamination. OWTSs effectively treat wastewater when located in suitable environmental conditions and regularly maintained. However, these criteria are not always met, and the system can become susceptible to failure resulting in excess nutrients and harmful pathogens entering the surrounding environment. More information is needed to understand if and how OWTSs are correlated to surface water contamination and how the correlation changes with watershed scale. Thus, the objectives of this study are to 1) model the susceptibility of OWTS failure throughout the Choccolocco Creek watershed, and 2) determine if there is a relationship between E. coli concentrations in surface water with both modeled OWTS failure and land cover type and determine if watershed scale affects results in the Choccolocco Creek watershed. The Choccolocco Creek, a tributary to the Coosa River, is on the Alabama 303(d) List of Impaired Waterbodies for elevated concentrations of E. coli, a fecal indicator bacterium. The source(s) and relative contributions of E. coli are unclear, and typical of mixed-use watersheds, it is difficult to identify E. coli source(s) without advanced chemical analyses. However, geospatial methods can assist in identifying potential sources by exploring the geographic relationships between source areas and E. coli concentrations. To address the first objective, GIS-based multi-criteria decision analyses (MCDA) were used to determine locations that have increased susceptibility to OWTS failure based on environmental variables (soil characteristics, proximity to surface water, and slope) and OWTS variables (age and density). With the model that included environmental and OWTS variables, an area of 44.3 km2 was identified as having high susceptibility to OWTS failure. Results indicate that OWTS age could be a driving factor of OWTS failure in the watershed. For the second objective, results from OWTS failure models and distribution of land cover type were correlated to E. coli concentrations measured monthly from April to September 2021 at nine water sampling locations along the Choccolocco Creek. The water sampling locations were used to delineate nine sample point watersheds (SPWs) and 63 distance derived watersheds (DDWs) to summarize land cover distribution and OWTS failure models and assess how results vary with watershed scale. The analysis yielded several key results: 1) significant, positive correlations were found between E. coli concentrations and OWTS failure models for both SPWs and DDWs; 2) a significant, positive correlation between E. coli and OWTS count was found. Additionally, variation in the significance of correlations differed with watershed scale, demonstrating the importance of selecting an appropriate unit of analysis. Our results suggest a relationship between OWTSs and elevated E. coli concentrations observed in Choccolocco Creek. Nonpoint source attribution challenges are not unique to the Choccolocco Creek watershed and methods outlined here could be applied to other watersheds to elucidate if OWTSs contribute to E. coli contamination of surface waters.