DEVELOPMENT AND APPLICATION OF A NUTRIENT ATTENUATION ASSESSMENT METHODOLOGY FOR CHESAPEAKE BAY WATERSHED ONSITE WASTEWATER TREATMENT SYSTEMS

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1 DEVELOPMENT AND APPLICATION OF A NUTRIENT ATTENUATION ASSESSMENT METHODOLOGY FOR CHESAPEAKE BAY WATERSHED ONSITE WASTEWATER TREATMENT SYSTEMS Victor A. D Amato, PE 1 ABSTRACT The United States Environmental Protection Agency (US EPA) Chesapeake Bay Program (CBP) created an Expert Panel to review the available science and provide recommendations on how to factor nutrient attenuation into Chesapeake Bay Total Maximum Daily Load (TMDL) onsite wastewater treatment system (OWTS) load estimates. In this context, "attenuation" is described as nutrient load reductions that occur between the OWTS drainfield and the receiving surface water. Specifically, the Panel revised the CBP s former assumptions of 20 percent total nitrogen (TN) reduction in the soil treatment unit and 60 percent attenuation of TN load between the system and modeled stream reach, incorporating spatially variable TN reductions depending on soil texture and underlying hydrogeomorphology. The Panel reviewed existing relevant literature on TN reductions within soil-based treatment systems, and supplemented this review with targeted modeling of TN reductions using the Soil Treatment Unit Model (STUMOD) to estimate TN reduction within the OWTS drainfield based on surficial soil textural class. The Panel also reviewed existing groundwater TN plume and load delivery case studies, and literature on nitrogen attenuation within the Chesapeake Bay watershed to establish a series of TN transmission classifications and attenuation factors for 15 distinct hydrogeomorphic regions (HGMRs) that span the entire watershed. Combining the soil treatment unit and groundwater attenuation factors resulted in a total of 12 separate TN attenuation rates ranging from 46 percent to 88 percent depending on OWTS location and characteristics. These revised TN attenuation rates - which represent TN load delivery to groundwater-surface water transitional zones (e.g., riparian areas, hyporheic zone) - have been adopted by the CBP and will be used in subsequent load analyses and crediting programs. The methodology developed by the Expert Panel is science-based, robust, vetted, and transferable to many other areas interested in assessing nutrient loading associated with OWTS. INTRODUCTION The Chesapeake Bay Watershed is under a well-publicized clean-up plan (or pollution diet ) driven by total maximum daily loads (TMDL) limits for several pollutants including the nutrients nitrogen and phosphorus, measured as Total Nitrogen (TN) and Total Phosphorus (TP). Onsite wastewater treatment systems (OWTS) are relatively modest pollution contributors to the Chesapeake Bay Watershed, estimated to contribute approximately 5 percent of the Bay s total TN load and zero percent of its TP load (i.e., 100 percent TP attenuation is currently assumed). 1 Associate Director. Tetra Tech. P.O. Box 14409, 1 Park Drive, Suite 200, Research Triangle Park, NC (919) victor.damato@tetratech.com.

2 The Wastewater Treatment Workgroup (WWTWG) of the Chesapeake Bay Program (CBP), a regional partnership that has led and directed the restoration of the Bay since 1983, has initiated several efforts to better understand and manage nutrients from OWTS in the past decade. A report on OWTS nitrogen reducing technologies or best management practices (BMPs) was approved by the CBP in February 2014 (Tetra Tech, 2014). This report was developed by an expert panel convened by the CBP and expanded the list of activities that Chesapeake Bay Watershed jurisdictions could use to get credit for reducing TN loading from OWTS, to include constructed wetlands, intermittent and recirculating media filters, and integrated fixed-film activated sludge (IFAS) systems, as well as dispersal area practices that improve TN removal, including shallowplaced, pressure dosed drainfields. This initial expert panel confirmed and clarified the CBP s baseline septic tank effluent (STE) TN load estimates, but recommended reviewing the CBP assumptions for treatment within the drainfield and between the drainfield and modeled surface waters (termed attenuation ), which are summarized in Figure 1. Fig. 1. CBP Baseline TN Load Assumptions for OWTS Delivered to Modeled Chesapeake Bay Stream Reaches (Tetra Tech, 2014) Accordingly, the OWTS TN Attenuation Expert Panel (the Panel) was formed in June of The Panel was tasked with recommending spatially-variable, scientifically-supported improvements to the assumptions summarized in Figure 1, which reflect a 20 percent TN reduction in the drainfield ( treatment ) and an additional 60 percent TN reduction between the drainfield and the edge of the modeled stream reach in the Chesapeake Bay water quality model ( attenuation ) for all OWTS within the Chesapeake Bay Watershed regardless of site characteristics or geographic location. Specifically, the Panel was charged with reviewing available science on how to factor nutrient attenuation into Chesapeake Bay TMDL onsite wastewater treatment system load estimates and BMP efficiency factors, including to:

3 Determine whether the Bay TMDL model can be improved by using variable total nitrogen (TN) attenuation rates Determine whether the currently used 100 percent removal of total phosphorus (TP) is warranted (note that the Panel was not able to consider TP reductions given the complexity of TN fate and transport and limited resources) Recommend methodologies to be used and specific attenuation rates to be used in different contexts The Attenuation Panel convened approximately 20 times between 2014 and Its final report was approved in October 2016 (Tetra Tech, 2016). The Panel included at least 17 members including representatives from Watershed states as well as academics and state and federal government representatives with expertise in OWTS, nutrient fate and transport, modeling, engineering, hydrogeology, and other relevant fields. In addition to the official Panel members, at least 30 other professionals contributed technical expertise to the Panel s work. METHODS The Panel s work included three main steps: Developing a conceptual framework for evaluating and communicating nutrient removal processes in OWTS. Conducting a literature review, which included a review focused on TN removal in onsite wastewater treatment, as well as a review of literature (mostly by USGS) on the hydrogeomorphology of the Chesapeake Bay Watershed and, in particular, the relative transmission of TN through the various geological formations present. Using modeling to corroborate findings from the literature. This included two existing mathematical models: o STUMOD (Soil Treatment Unit Model), developed by the Colorado School of Mines, is a spreadsheet model that uses analytical solutions to simulate the steady state distribution of water content, ammonium concentration, and nitrate concentration in the unsaturated zone beneath an OWTS (Geza et al., 2013). o SPARROW (Spatially Referenced Regression on Watershed Attributes) is a modeling tool developed by the USGS for the regional interpretation of waterquality monitoring data (USGS, 2016). The model relates in-stream water-quality measurements to spatially referenced characteristics of watersheds, including contaminant sources and factors influencing terrestrial and aquatic transport. SPARROW empirically estimates the origin and fate of contaminants in river networks and quantifies uncertainties in model predictions. The Panel recognized the limitations in the state-of-knowledge and the need to consider multiple sources of information and lines of evidence in support of their findings and recommendations (i.e., a weight of evidence approach). The Panel also recognized that their recommendations were intended to apply mainly to the representation of baseline conditions in the CBP water quality model; therefore, the assessment focused on characterizing average conditions while identifying performance ranges, where applicable.

4 RESULTS As an initial step, the Panel found it important to develop a mechanistic framework for conceptualizing and describing the fate and transport of nutrients associated with OWTS. This framework is summarized in Figure 2 and includes four distinct zones where nitrogen transformations are expected to occur. Fig 2. OWTS Treatment and Attenuation Zones (Tetra Tech, 2016) Zone 1, the Soil-Based Treatment Zone, is the unsaturated zone extending 30 to 60 cm below the infiltrative surface. Its precise boundary can be better located using biogeochemical measurements that indicate enhanced biological activity affected by the application of STE or treated effluent. These measurements will differ from those in Zone 2, the bulk Vadose Zone, where such measures will be similar to background levels. The boundaries of Zone 1 are analogous to the CBP s edge of drainfield concept within which a 20-percent TN reduction had been assumed for all OWTS in the watershed. Nitrification is a particularly important function of Zone 1 in most cases. The Panel s approach to spatially differentiating TN reductions in Zone 1 was based on the texture of the surficial soils with coarser textured soils providing lower reductions and finer textured soils higher reductions. Zone 2 was considered exceptionally difficult to spatially define on a large watershed basis (considering site scale heterogeneities, fluctuating water tables, and the like), and relatively unimportant in terms of TN transformation compared with other zones. Therefore TN reductions in Zone 2 were not considered in the Panel s analysis. Zone 3 represents the groundwater zone, which is typically characterized by predominantly horizontal flow toward an outlet (e.g., stream) and long residence times compared with the other zones. Although denitrification rates in Zone 3 are typically low and limited by a lack of organic carbon, travel times can be high enough to effect significant TN reductions. For this effort, TN

5 reductions in Zone 3 were related to the underlying hydrogeomorphology, with no direct consideration given to relative travel time within a given geomorphological area. Zone 4 is a catch-all for transitional zones at the groundwater-surface water interface. It includes floodplain and riparian area transport, transport through the hyporheic zone, and attenuation that occurs in small streams of a lower order than the surface waters simulated in the Chesapeake Bay water quality model. Although it was recognized that significant TN reductions of more than 50 percent have been documented in transitional areas, other CBP efforts are considering these landscape-scale impacts on nutrient delivery. So as to not duplicate efforts, the Panel did not consider Zone 4 TN reductions. The various OWTS components that could affect TN are summarized in Table 1, along with a description of the approach used by the Panel to estimate TN load reductions. As indicated, the Panel focused on Zone 1 (the soil-based treatment zone) and Zone 3 (the groundwater zone) in their recommendations. Table 1. OWTS Component and TN Reduction Approach (Tetra Tech, 2016) Component Comment Exsitu unit 1 (e.g., septic tank) No TN reduction assumed in septic tank (e.g., TN = 5 kg/person/year) Exsitu unit 2 (e.g., intermittent sand filter) Insitu Zone 1 (Soil-Based Treatment) Insitu Zone 2 (Vadose Zone) Insitu Zone 3 (Groundwater Zone) Insitu Zone 4 (Transitional Zones) TN reductions based on CBP approved BMP credits Varies by soil texture, based on STUMOD and field observations Assumed low in comparison to Zones 1 and 3; not explicitly addressed by Panel Varies by physiography and hydrogeology, informed by SPARROW model and field observations Small stream and riparian processing being addressed by other CBP efforts Zone 1 TN reductions were estimated by using STUMOD, corroborated with field data from the Chesapeake Bay Watershed and Southeast U.S. The results of the STUMOD runs are summarized in Table 2. Four scenarios were tested for each of the twelve USDA Soil Texture classes. The scenarios varied the depth below the infiltrative surface, and the actual hydraulic loading rate simulated. The 30 cm depth is consistent with a 12-inch vertical separation distance to groundwater or other unsuitable soil feature, while the 60 cm depth is consistent with a 24-inch separation distance. In practical terms, the 30-cm distance might better represent an area with a high proportion of legacy OWTS, installed prior to the advent of modern regulations and design standards. The 60-cm distance better reflects areas featuring well-designed modern systems. Likewise, the actual hydraulic loading rate was varied between 50 percent and 100 percent. Because most systems are not operated at their design flow on a consistent basis (design flows typically represent at least 90 th percentile daily flow rates), a 50-percent average actual loading is more realistic for well-designed systems, while a 100-percent average actual loading rate might better represent an undersized legacy system. For various reasons, the 60-cm separation, 100- percent actual loading rate simulation results were selected as most representative. The percent

6 reductions provide a mid-point between worst case (30 cm/100 percent) and best case (60 cm/50 percent) scenarios and the reductions matched up well with field data. As suggested in Table 2, individual TN reduction rates for the 12 USDA textures were averaged and rolled up into three major textural classes: Sandy, Loamy and Clayey. Figure 3 shows how surficial soil texture varies across the Chesapeake Bay Watershed. Table 2. STUMOD Results showing Zone 1 TN Reduction Recommendations (Tetra Tech, 2016) Soil textural class Loading rate (cm/day) TN reduction for a specified depth to groundwater and actual hydraulic loading rate applied 30 cm/100% 30 cm/50% 60 cm/100% 60 cm/50% Sand 4 (1.00 gpd/sf) 7% 16% 16% 31% Loamy sand Sandy loam Loam 4 (1.00 gpd/sf) 3 (0.75 gpd/sf) 3 (0.75 gpd/sf) Silt loam 1.8 (0.45 gpd/sf) 11% 30% 34% 59% Clay loam Sandy clay loam Silty clay loam Silt 1.8 (0.45 gpd/sf) 1.8 (0.45 gpd/sf) 1.8 (0.45 gpd/sf) 1.8 (0.45 gpd/sf) Sandy clay 1 (0.25 gpd/sf) 29% 54% 54% 80% Silty clay Clay 1 (0.25 gpd/sf) 1 (0.25 gpd/sf) Note: gpd/sf = gallons per day per square foot Fig. 2. Surficial Soil Texture across Chesapeake Bay Watershed (Tetra Tech, 2016) For Zone 3, the Panel reviewed existing literature on groundwater TN plume and load delivery case studies, and nitrogen attenuation by Chesapeake Bay hydrogeomorphic region to establish a

7 series of TN transmission classifications with associated Zone 3 attenuation factors for 15 distinct hydrogeomorphic regions (HGMRs) that span the entire watershed (Fig. 4). Recommended Zone 3 attenuation factors are summarized in Table 3. Fig. 4. Hydrogeomorphic Regions in the Chesapeake Bay Watershed. Map on left provides major regions (Bachman et al., 1998). Map on right further refines the coastal plain (Ator et al., 2005) Table 3. Recommended Zone 3 Attenuation Factors for Chesapeake Bay HGMRs (Tetra Tech, 2016) Hydrogeomorphic Region 1 Relative TN Transmission Classification Recommended Zone 3 Attenuation Factor (Transmission Factor) Fine Coastal Plain - Coastal Lowlands Low 75% (25%) Fine Coastal Plain - Alluvial and Estuarine Valleys Low 75% (25%) Fine Coastal Plain - Inner Coastal Plain - Upland Sands and Gravels Medium 60% (40%) Fine Coastal Plain - Middle Coastal Plain Mixed Sediment Texture Medium 60% (40%) Fine Coastal Plain - Middle Coastal Plain Fine Sediment Texture Low 75% (25%) Coarse Coastal Plain - Middle Coastal Plain Sands with Overlying Gravels (also dissected) High 45% (55%) Coarse Coastal Plain - Inner Coastal Plain - Dissected Outcrop Belt High 45% (55%) Crystalline Piedmont High 45% (55%) Crystalline Blue Ridge High 45% (55%)

8 Carbonate Piedmont Very High 35% (65%) Carbonate Valley and Ridge Very High 35% (65%) Carbonate Appalachian Plateau Very High 35% (65%) Siliciclastic Mesozoic Lowland High 45% (55%) Siliciclastic Valley and Ridge Medium 60% (40%) Siliciclastic Appalachian Plateau Low 75% (25%) 1 Generalized Geology from Greene et al. (2005); Subdivisions from Bachman et al. (1998), and Ator et al. (2005) for coastal plain A summary of the Panel s combined Zone 1 and Zone 3 recommendations is provided in Table 4, which shows the total recommended TN load for all possible combinations of soil textural classification (Zone 1) and TN transmission classification (Zone 3). These recommendations reflect the TN load delivered to the edge of Zone 4 (Transitional Zones) if present. Table 4. Recommended TN Load Delivery Rates at Outer Edge of Zone 3 (i.e., to Zone 4) (Tetra Tech, 2016) Soil Textural Classification Sandy Loamy Clayey USDA Soil Textures Sand, Loamy Sand, Sandy Loam, Loam Silt loam, Clay Loam, Sandy Clay Loam, Silty Clay Loam, Silt Sandy Clay, Silty Clay, Clay Note: kg/per/yr = kilograms per person per year DISCUSSION Low TN Transmission Area Medium TN Transmission Area High TN Transmission Area Very High TN Transmission Area 1.1 kg/per/yr 1.7 kg/per/yr 2.3 kg/per/yr 2.7 kg/per/yr 0.8 kg/per/yr 1.3 kg/per/yr 1.8 kg/per/yr 2.1 kg/per/yr 0.6 kg/per/yr 0.9 kg/per/yr 1.3 kg/per/yr 1.5 kg/per/yr It is exceptionally difficult to confidently estimate the surface water loadings of pollutants associated with OWTS (e.g., compared to point source measurements). Nevertheless, sewering projects are routinely justified based on an assumption that OWTS are to blame for local water quality impairments. To ensure that OWTS improvement or replacement projects are indeed warranted and to better understand and manage water quality, scientifically robust protocols for quantifying OWTS pollutant contributions are desperately needed. As the first estuary in the nation to be targeted for an integrated watershed and ecosystem restoration and one of the largest water quality restoration efforts ever, the Chesapeake Bay Program provides a model for other regions and large-scale water quality improvements. Guidance from the CBP continues to be adopted in jurisdictions within and outside of the watershed. Although the OWTS Nitrogen Attenuation Panel s recommendations necessarily rely on generalizations given the relative paucity of data compared with the large number of potentially important controlling variables (e.g., geology, soils, slope, hydrology, vegetation, installation depth, effluent characteristics), the protocol is sound and can be used as a model for other

9 watersheds. Indeed, by considering appropriate pollutant transformation mechanisms, the framework illustrated by Figure 2 and applied for this project can easily be adapted for other geologies as well as other pollutants that might be of interest, including TP and fecal indicator bacteria. Furthermore, the results from the Chesapeake Bay exercise, along with regional data, suggest that TN reductions can range up to about 90 percent, with additional significant removals likely in surface water-groundwater transitional areas (i.e., Zone 4). The recommendations summarized in the Results section are generally applicable to modern conventional OWTS in the Chesapeake Bay watershed, although some conservatism was built into Zone 1 estimates to account for OWTS performing suboptimally. However, the Panel did not explicitly discriminate between modern systems and legacy systems (those installed before modern standards that emphasize treatment in the soil rather than focusing on effluent disposal). As previously mentioned, the findings and recommendations should generally be taken to represent average systems within the specified context (i.e., surficial soil texture for Zone 1, hydrogeomorphic region for Zone 3), and care should be taken when using the findings to draw inferences about specific individual systems or in areas known to include an unusually high percentage of legacy or malfunctioning systems. Accordingly, applying this methodology is warranted as a first step to estimate the magnitude of the pollutant load associated with OWTS and its relative importance with respect to water quality impairment, considering other known sources. The outputs of such an analysis should reveal areas where OWTS are likely to deliver greater pollutant loads relative to other areas and thus can provide unique insight into prioritizing OWTS for improvements (e.g., improved management, the use of advanced OWTS, cluster systems, or sewering). Such spatially-differentiated load estimates can be a fundamental component of a larger prioritization process that utilizes a more complete set of OWTS inventory data (e.g., system type, system age, malfunction history, distance from impaired waters, other soil/site characteristics). At its most refined spatial scale (e.g., neighborhood scale), field inspections, targeted ground and surface water quality sampling, stream surveys, infrared photography flyovers, and other techniques can be used to confirm inferences and select appropriate OWTS management strategies to minimize environmental and public health impacts. Figure 5 provides a summary of the results of a typical large-scale OWTS inventory, prioritization, and management selection process for the State of Maryland s Chesapeake Bay Watershed OWTS (Tetra Tech, 2011).

10 Fig. 5. State of Maryland OWTS Prioritization and Management Planning Map (Tetra Tech, 2011) CONCLUSIONS AND RECOMMENDATIONS The Chesapeake Bay Onsite Wastewater Total Nitrogen Attenuation Expert Panel developed new, spatially refined TN removal rates for onsite wastewater systems throughout the Chesapeake Bay watershed. The rates generally apply to properly functioning conventional onsite wastewater systems and use surficial soil texture and underlying hydrogeomorphology to inform estimates of TN load transmission and reduction through the active soil treatment unit and the groundwater, respectively. The Panel s recommendations provide a more robust way to represent nutrient loads from the onsite sector in the Chesapeake Bay water quality model and help target management efforts where they are most needed. Importantly, the Panel s conceptual framework and related load estimation methodologies are broadly applicable to other geographies addressing nutrient loading believed to be affected by onsite systems (e.g., Cape Cod, Long Island) as well as geographies addressing other pollutants (e.g., fecal indicator bacteria) believed to be affected by onsite systems (e.g., Russian River, California). The former application mostly requires an understanding of the hydrogeomorphological features of the areas in question and, in particular, how they impact nutrient transmission. The latter application requires reconsideration of how fate and transport

11 processes in Zones 1 through 4 affect the pollutant(s) in question. Nevertheless, the framework provides a logical way to tackle the complex issues inherent to attributing pollutant loads to nonpoint sources like onsite wastewater systems. Although the Panel s work represents a substantial advance in OWTS pollution load estimation methodologies, many questions remain to be answered. Accordingly, future efforts should focus on the following. Improving understanding of the factors affecting nutrient processing by conducting additional, deeper literature and existing data reviews and by collecting new empirical and modeling data, including collecting more data about existing systems and sites within the Chesapeake Bay watershed. Addressing phosphorus treatment and attenuation. Explicitly differentiating between conventional OWTS, and malfunctioning and legacy systems. Reducing malfunctions and upgrading legacy systems could be considered as future best management practices. The time distribution of load delivery including understanding long-term system lags that might impact nutrient loading dynamics, short-term nutrient load delivery dynamics (e.g., how does load delivery relate to baseflow and stormflow conditions), and travel time with respect to Zone 3 TN load reduction estimates. The results of pollutant load estimation exercises informed by the Attenuation Panel s work can be used to: More accurately attribute pollutant loads to sources, including onsite systems; Help identify problematic systems or areas that should be targeted for management; and Inform appropriate management strategies by elucidating the particular mode of pollutant transport to receiving waters. ACKNOWLEDGMENTS At least forty-seven people contributed to the report upon which this paper is based and all of their assistance is greatly appreciated. Special thanks are extended to the Panelists and coauthors, including Lewis Linker, Ning Zhou, and David Wood, Chesapeake Bay Program Office; Steven Berkowitz, North Carolina Dept. of Health and Human Services; Tom Boekeloo, New York State Dept. of Environ. Conservation; Jay Conta, Virginia Tech/Virginia Dept. of Health; Marcia Degen, Virginia Dept. of Health; Judy Denver, United States Geological Survey; Joshua Flatley, Maryland Dept. of Environment; John Galbraith, Virginia Tech; Barry Glotfelty, Frederick County (MD) Health Department; Robert Goo, US EPA - Office of Watersheds, Oceans and Wetlands; Jack Hayes, Delaware Dept. of Natural Resources and Environmental Control; George Heufelder, Barnstable County (MA) Department of Health and Environment; Dave Montali, West Virginia Department of Environmental Protection; Michael O'Driscoll, East Carolina University/Duke University; David Radcliffe, University of Georgia; Eberhard Roeder, Florida Department of Health; and Robert Siegrist, Colorado School of Mines.

12 LITERATURE CITED Ator, S.W., J.M. Denver, D.E. Krantz, W.L. Newell, and S.K. Martucci A surficial hydrogeologic framework for the Mid-Atlantic Coastal Plain. Professional Paper Bachman, L.J., B.D. Lindsey, J.W. Brakebill, and D.S. Powars Ground-water discharge and base-flow nitrate loads to non-tidal stream, and their relation to a hydrogeomorphic classification of the Chesapeake Bay watershed, Middle Atlantic Coast: USGS WRIR , 77 p. Geza, M.N., K. Lowe, and J. McCray STUMOD A tool for predicting fate and transport of nitrogen in soil treatment units. Environmental Modeling & Assessment, 19(3), Greene, E.A., A.E. LaMotte, and K. Cullinan Ground-water vulnerability to nitrate contamination at multiple thresholds in the Mid-Atlantic region using spatial probability models: USGS SRI Tetra Tech Decentralized wastewater management gap closer research and analysis. Prepared for Maryland Department of the Environment (Baltimore, Maryland) and available at GapCloserResearchandAnalysis.pdf. Prepared by Victor D Amato, Tetra Tech. March Tetra Tech Recommendations of the On-Site Wastewater Treatment Systems Nitrogen Reduction Technology Expert Review Panel final report. Submitted to Wastewater Treatment Workgroup, Chesapeake Bay Partnership, US EPA and available at reatment_systems_expert_panel. Prepared by Victor D Amato, Tetra Tech. Tetra Tech Nutrient attenuation in Chesapeake Bay Watershed onsite wastewater treatment systems - final report. Chesapeake Bay Onsite Wastewater Nutrient Attenuation Expert Review Panel. Submitted to Wastewater Treatment Workgroup, Chesapeake Bay Partnership, US EPA and available at Prepared by Victor D Amato, Tetra Tech. USGS SPARROW surface water-quality modeling. United States Geological Survey (USGS). Accessed August 15, 2016.