Refined Model Provides a Screening Tool to Understand Exposure to Contaminants from Incidental Wastewater Reuse

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Refinement of the existing national-scale “de facto reuse incidence in our nation’s consumable supply” (DRINCS) model, complemented by field measurements, provides a screening tool to understand human and wildlife exposure to toxicants and pathogens associated with the incidental reuse of treated wastewater in the Shenandoah River watershed. The model results can be accessed in a companion web-based mapping application.

Two Mobile Fish Exposure Laboratories with Inset Showing the Inside of One

Scientists deployed mobile fish exposure laboratories, such as these two, at sites in the Shenandoah River watershed, Virginia, to understand exposure and effects on fish. Inside the laboratories (see inset photo) fish were placed in aquariums, and exposed to stream water or wastewater during the study.

(Credit: Jennifer L. Rapp, U.S. Geological Survey, Virginia and West Virginia Water Science Center. Public domain.)

Reuse of treated wastewater discharged from municipal and industrial sources can be either intentional, such as in a system where potable water is recycled and reused, or incidental, when treated wastewater is discharged into surface waters to maintain freshwater supplies. This incidental reuse is commonly referred to as de facto reuse and can become a part of the source water taken in by drinking water facilities.

Incidental reuse can be a challenge because domestic and industrial wastewaters often contain complex mixtures of chemicals such as disinfectant by product precursors, steroid hormones, pharmaceuticals, fire suppressants, and personal care products that are not completely removed during treatment. Complex chemical mixtures have been documented in streams and, in some cases, in the source waters of drinking water facilities.

As incidental wastewater reuse increases, there is an increasing interest in the quality and availability of surface-water resources as they relate to wildlife and human health. Ecological resource managers have an interest in incidental reuse because treated wastewater often contains toxicants that can affect resident biota. Drinking water facility managers have an interest in incidental reuse because it may affect the occurrence of toxicants in their source waters.

The integrated science teams of the U.S. Geological Survey (USGS) Environmental Health Program (Toxic Substances Hydrology and Contaminant Biology), in collaboration with Arizona State University, University of North Carolina Charlotte, and George Mason University, refined an existing model to create a screening tool to understand and predict the likelihood of contaminants from upstream wastewater discharges being present at downstream locations, including surface-water intakes at drinking water facilities.

Clean drinking water

Incidental reuse can be a challenge because domestic and industrial wastewaters, at times, contain complex mixtures of chemicals. Complex chemical mixtures have been documented in the source waters of drinking water facilities.

(Credit: Barbara Mahler, USGS. Public domain.)

The scientists selected the 7,920-square-kilometer Shenandoah River watershed in northern Virginia and eastern West Virginia to build on previous studies. The watershed consists of a diverse landscape; serves as the source for drinking water for more than 200,000 people; has 13 drinking water facilities that utilize surface water as a source, 22 stream gages with long-term records, and 81 wastewater treatment and 25 industrial discharge locations that met study parameters.

As a first step, the team refined the existing national-scale “de facto reuse incidence in our nation’s consumable supply” (DRINCS) model, which was designed for the conterminous United States, so that the model was more applicable to the smaller-scale Shenandoah River watershed. The refined model included communities less than 10,000 people, incorporated municipal and industrial wastewater treatment facilities, and was used to calculate the fraction of streamflow consisting of wastewater effluent, called the accumulated wastewater in percent of streamflow (ACCWW%). The ACCWW% was calculated using monthly and annual 2015 wastewater discharge data reported by the wastewater treatment facilities for two streamflow conditions (mean annual and mean August from 1971 to 2000) estimated from the National Hydrography Dataset Plus V2 Enhanced Run-off Model.

Determining the ACCWW% in a stream is an essential first step to assessing chemical exposure pathways and potential risks of wastewater reuse for wildlife and humans. During the period of the study, the ACCWW% under mean-annual streamflow ranged from 0 to 17 percent among the 1,754 river segments analyzed. At surface-water intakes within drinking water treatment plants, the ACCWW% ranged from less than 0.01 to 2.0 percent under mean-annual streamflow and as much as 4.5 percent during August streamflow. The ACCWW% was then utilized in two separate studies to understand wildlife and human exposure scenarios.

The first study focused on developing tools to understand and predict endocrine disrupting chemical (EDC) occurrence and effects on fish. The scientists used the ACCWW% to calculate predicted concentrations of selected EDCs and estradiol potency factors. The predicted and measured estradiol potency factors and the actual concentrations of individual EDCs were less than 1 nanogram per liter at 98 percent of the stream reaches in the watershed, indicating that under the hydrologic conditions used in the model, wastewater-derived endocrine disruptors were not at concentrations that would pose a substantial risk to resident fish. Complementary, 21-day mobile-laboratory experiments in 2014–16 exposed adult fathead minnows (Pimephales promelas) to stream waters at eight sites in close proximity upstream and downstream from two municipal wastewater treatment plants and at one reference site with minimal wastewater discharge in the watershed. The results from the exposures also indicated low-to-moderate risk of fish endocrine disruption.

View of the Shenandoah River

The Shenandoah River

(Credit: Jennifer L. Rapp, U.S. Geological Survey, Virginia and West Virginia Water Science Center. Public domain.)

The second study focused on understanding the effect of upstream ACCWW% on the formation and occurrence of disinfection byproducts (DBPs), such as trihalomethanes and haloacetic acids, in downstream drinking water facilities to understand human exposure through drinking water. Using the modelled ACCWW% and measured DBP concentration data reported by the drinking water facilities, the scientists noted differences in DBP formation among drinking water facilities. Those differences were the most notable during August when ACCWW% was the greatest. Concentrations of DBPs in finished water at facilities that utilize stream water for their source increased as ACCWW% increased. This finding indicates that organic precursors that lead to DBP formation are higher in streams impacted by higher percentages of de facto reuse. A 1 percent or greater level of de facto reuse near the drinking water facility intakes was associated with greater concentrations of DBPs under mean-annual streamflow conditions.

Wastewater reuse is an important component of the hydrological cycle and affects water availability and water use suitability. The two studies described here provide screening level tools to understand and predict contaminant occurrence for the Shenandoah River watershed. The screening tools can be adjusted for streamflow and wastewater inputs to understand exposure scenarios related to human population growth, wastewater infrastructure upgrades, changes in land-use practices, climate change, and more. Although this work was focused on DBPs and EDCs, the methods apply to a broad range of contaminants and biological end points and can be refined for use in other watersheds.

Screenshot of Shenandoah River Watershed Accumulated Wastewater Ratio Mapper

Screenshot of the Shenandoah River Watershed Accumulated Wastewater Ratio Mapper—a web-based mapping application. The mapping application provides a tool to view accumulated percent of wastewater at designated river segments (color coded stream segments), wastewater and industrial treatment plant discharges (red and green circles respectively), and U.S. Geological Survey stream gaging stations (black triangles). 

(Credit: Virginia and West Virginia Water Science Center. Public domain.)

The integrated science teams of the Environmental Health Program (Toxic Substances Hydrology and Contaminant Biology) are moving forward to reduce uncertainties in the modelling approach by including better transport and fate information and the ability to account for other sources of contaminants including runoff from urban and agricultural lands. The teams are also working to determine if the models can be extended to incorporate effects of exposure to aquatic organisms such as fish.

The Toxic Substances Hydrology and Contaminant Biology combined programs supported these studies.