Tracking land-based sources of nutrients and microbial contamination in a Pacific Northwest estuarine watershed

Tillamook Bay (Oregon, USA) often experiences water quality impairments due to low oxygen levels and elevated fecal bacteria counts. Potential pollutant sources in this mixed-use watershed include agricultural (extensive dairy operations in lowlands), wildlife, and human waste (municipal and onsite wastewater treatment systems), making fecal pollution management challenging. In this study, a multiple water quality metric approach was used to characterize fecal indicator bacteria (FIB), microbial source tracking (MST) genetic markers, and nutrients in surface waters. Covariation in nutrients, FIB, and MST marker levels was investigated. Water samples were collected from 16 tributary sites, above and below potential anthropogenic pollution sources on all five rivers draining into Tillamook Bay. Paired measurements of MST markers indicating human (HF183/BacR287 and HumM2), ruminant (Rum2Bac), cattle (CowM2 and CowM3), canine (DG3), and avian (GFD) fecal pollution sources were compared to parallel nitrate isotopic composition (δ15NNO3) and E. coli (MPMN/100mL) measurements. In tributaries, seasonal and spatial patterns were evident, with cow and human MST marker levels increasing along a downstream gradient. During the wet season, ruminant marker levels were highly correlated with E. coli (r2 = 0.89) and nitrate isotope (r2= 0.83). Nitrogen load estimates from SPARROW demonstrate that manure and fertilizer explain >80% of the variation in ruminant marker, E. coli, and nitrate isotope levels. Results support the utility of using a combined water quality metric approach for tracking land-based pollutant sources in complex watersheds.

Impact/Purpose

Nutrients often enter water bodies with other co-pollutants such as bacteria. Tools are needed to identify pollutant sources within particular water bodies so that remediation efforts can be directed to the appropriate source to maximize the effectiveness of remediation efforts. Water samples were collected monthly (Jul 2016 - May 2017) at 16 tributary sites of the Tillamook Estuary, a system that has point source inputs associated with wastewater treatment facilities, and nonpoint source inputs associated with agricultural operations and on-site septic systems. We demonstrate that additional discrimination of sources can be obtained by combining microbial source tracking, stable isotopes, and watershed models of nitrogen inputs. Through this approach we were able to identify which sites are influenced by agricultural inputs, point source inputs, as well as unexpected contributions from birds.

Citation

Zimmer-Faust, A., Cheryl A. Brown, Jim Kaldy, TChris MochonCollura, O. Shanks, W. Rugh, P. Clinton, H. Stecher, A. Manderson, AND Y. Johnson. Tracking land-based sources of nutrients and microbial contamination in a Pacific Northwest estuarine watershed. Virtual National Recreational Water Quality Workshop, NA, Virtual, April 06 - 08, 2021.