Using Hydrology as a Surrogate in TMDL Development for Impairments Caused by Multiple Stressors

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Using Hydrology as a Surrogate in TMDL Development for Impairments Caused by Multiple Stressors Thambirajah Saravanapavan*1, Eiji Yamaji2, Mark Voorhees3, and Guoshun Zhang4 Center for Water Resources, Tetra Tech, Inc., Fairfax, Virginia, USA

1,4

Graduate School of Frontier Sciences, The University of Tokyo, Tokyo, JAPAN

2

U.S. Environmental Protection Agency, Region 1, Boston, Massachusetts, USA

3

tham.saravanapavan@tetratech.com; 2yamaji@k.u‐tokyo.ac.jp; 3mark.voorhees@epa.gov; guoshun.zhang@tetratech.com

*1 4

Received 10 September 2013; Accepted 14 October 2013; Published January 2014 © 2014 Science and Engineering Publishing Company

Abstract

Introduction

Habitat and aquatic life impairments are unfortunate consequences of the urbanization process in many places. In the United States, total maximum daily loads (TMDLs), restoration strategies or plans, are developed to help maintain water quality standards. The development of TMDLs to mitigate habitat and aquatic life impairments, however, presents unique challenges as the impairments are often associated with multiple stressors. While the stressors may vary from specific pollutants, stormwater runoff, hydrologic modifications, riparian corridor encroachment, and channel alteration, the determination of the exact role and relative significance of individual stressors leading to the impairment and the subsequent development of TMDLs for corresponding pollutants are often technically and financially challenging. An innovative approach of using hydrology as a surrogate in developing TMDLs for mitigating habitat and aquatic life impairment caused by multiple unspecified stressors was introduced in this study.

The approach directly addresses the ultimate driving force of many stressors (pollutant loading, habitat destruction and hydrologic alteration) causing habitat and aquatic life impairment, namely the excess stormwater runoff.

Applications of the innovative approach for quantifiable habitat and aquatic life TMDL development have been demonstrated through two case studies, one in the Shawsheen watershed of Massachusetts, USA and the other in the Indian Brook watershed of Vermont, USA. The results indicated that the approach can effectively facilitate the development of TMDLs for impairment situations characterized by multiple unknow stressors and limited data.

Natural river flow conditions around the world have been negatively impacted as the trend of urban sprawl continues to accelerate (Chen and Wu, 1987; Sparks, 1992; Dynesius and Nilsson, 1994; Walker et al., 1995).

Changes in flow conditions directly influence river biota, especially the habitat and aquatic life (Ward and Stanford, 1979, Petts, 1984; Calow and Petts, 1992). For example, organisms that are sensitive to flow velocity, including periphyton, phytoplankton, macrophytes, macroinvertebrates, young fish and deposited eggs, are easily displaced as a result of increased frequency or duration of high flow levels (Moog, 1993; Allan, 1995). Channelization and dredging, as well as riparian vegetation elimination all modify the habitat (OEPA, 2012). At the same time, pollutants discharged into rivers and other receiving water bodies also cause adverse biological effects: infection of organisms by bacteria and viruses, increased biological oxygen demand (BOD) and lower dissolved oxygen (DO), death from chronic toxicity exposure and alteration to natural habitat cycles and breeding, etc. (Zoppou, 2001; Whitehead et al. 2006). In fact, habitat alteration and impaired biota have been the leading causes of impairment in assessed United States rivers and streams for many consecutive years, regardless of designated uses (USEPA, 2007, 2009).

Keywords

In the United States, the total maximum daily load (TMDL) program has been established by the Clean Water Act (CWA) to address water quality issues in

Urbanization; TMDL; Surrogate Indicators; Stormwater Management; Aquatic Life Impairments; Multiple Stressors

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the nation’s waters. The development of TMDL for a specific water body involves identifying the pollutant/stressor causing the water quality impairment, estimating the amount of pollutant that the water body can assimilate, calculating pollutant loadings from various sources, determining allowable pollutant load based on analysis of pollution in the water body, and finally allocating pollutant loads with a margin of safety (MOS) (Wagner et al., 2006). The TMDLs are usually developed using either or both of the U.S. Environmental Protection Agency (USEPA) approved methods: load duration curves (LDCs) and/or water quality models (USEPA, 2007). While load duration curves are often applied to small watersheds where flow is the primary mechanism in pollutant loading, water quality models are more appropriate in analyzing larger, more complex watersheds where watershed‐scale pollutant transport models are needed (Sakura‐Lemessy, 2009). Since the inception of the TMDL program in the 1970s, a total number of 39,478 non‐point source TMDLs has been developed for various pollutants (e.g. pathogens, sediment, nutrients, heavy metals, temperature, bacteria, etc.) around the U.S. in addressing various water quality issues (USEPA, 2011).

Despite wide‐spread implementations of the TMDL program, limited numbers of TMDLs were developed to directly address the habitat and aquatic life impairments. This is mainly due to the fact that evaluations of interrelationships between flow phenomena and biotic responses can be time consuming due to long‐term monitoring and analysis needs, and this is further complicated by the fact that habitat and aquatic life impairments are often caused by multiple stressors. The stressors could vary from known and unknown pollutants to storm water runoff, hydrologic modifications, riparian corridor encroachment, and to channel alteration (NRC, 1992; Richter, 1997). The determination of exact role and relative significance of each pollutant/stressor in contributing to the impairment could easily exceed the technical and fiscal resources of many communities.

As practical TMDL implementation plans require reduction targets for specific stressor(s), the lack of definitive relationship between habitat and aquatic life impairments and corresponding stressor(s) limits the development of traditional TMDLs for water quality protection (OEPA, 2012).

A comprehensive review of water quality cannot be achieved without accounting for changes in hydrological regimes. Although many are not directly

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initiated through TMDL efforts, a growing body of scientific literature has endorsed the “natural flow paradigm” for meeting ecological management targets in rivers and streams (Richter et al., 1997; Poff et al., 1997; Bunn and Arthington, 2002; Postel and Richter, 2003; Arthington et al., 2006; Richter, 2009). The objective for the approach is to restore native aquatic biodiversity and ecosystem integrity through maintaining or restoring some semblance of natural flow variability (Richter et al., 1996; Stanford et al., 1996; Richter et al., 2011). The key underlying argument for the approach is that hydrological variations play a major part in structuring the biotic diversity within ecosystems, mainly through the controlling of key habitat conditions within the river channel, the floodplain, and hyporheic (stream‐ influenced groundwater) zones (Poff and Ward, 1989; Arthington and Pusey, 1994; Townsend and Hildrew, 1994; Stanford et al., 1996). Moreover, the exchanges of organisms, particulate matter, energy, and dissolved substances along the upstream‐downstream, river‐ floodplain, river‐hyporheic, and temporal dimensions of riverine ecosystems are substantially mediated by flow conditions including stream flow, floodplain inundation, alluvial groundwater movement, as well as water table fluctuations (Ward and Stanford, 1983, 1995; Ward 1989; Sparks et al., 1990; Walker et al., 1995; Richter et al. 1997).

An innovative approach in which the natural hydrology is regarded as surrogate for developing TMDLs for habitat and aquatic life impairments caused by unspecified sources is introduced in this study. The approach builds on existing body of knowledge regarding flow regime and ecological integrity, and incorporates the attainment watershed approach during the TMDL development. With the specific goal of developing TMDLs for habitat and aquatic life impairments caused by multiple stressors, the innovative approach is illustrated in two New England watersheds: the Shawsheen watershed in Massachusetts and the Indian Brook watershed in Vermont.

Methodology Historical studies linking hydrological parameters and stream ecological integrity provide references for choosing surrogates for habitat and aquatic life impairment assessments. Arthington et al. (1991) evaluated four attributes of natural flow regime in drawing flow recommendations in Australia, and the attributes include low flows, first major wet‐season

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used ways to display the distribution of stream flows under either natural or regulated conditions (Gordon et al. 1992). A sample flow duration curve is illustrated in Figure 1. As shown in the figure, the high and low ends of the curve give indications of the flooding and base flow conditions in the stream. The shapes of the curves are signatures of flow characteristics of a stream for comparisons within or between catchments.

The curve can also serve as the basis for generating duration curves of sediment, turbidity, water hardness, or other water quality characteristics, provided that a relationship is established between stream flow and the characteristic of interest (Gordon et al., 1992). That is, the FDC for a stream provides the potential linkage between measurable hydrologic parameters (e.g. flow rates at different exceedance levels) and corresponding water quality situations which are directly connected to attainment of designated uses.

flood, medium sized floods, and very large floods. The flow target for maintaining natural flow regime was set as the lowest flow that occurred often (e.g. estimated as a specified percentile exceedance flow for each month). Richter et al. (1997) proposed the Range of Variability Approach (RVA) to help conserve native aquatic biodiversity and protect natural ecosystem functions in rivers and streams. The approach recognizes the fact that human‐induced flow alterations can both deplete and unnaturally augment natural flows to the detriment of ecological health. As a result, the approach subsequently establishes the band of allowable alterations called “sustainable boundaries” around natural flow conditions as a means of expressing environmental flow needs. The extent to which the hydrologic variability, as well as the associated characteristics of timing, frequency, duration, and rates of change, helps sustain aquatic ecosystems was analyzed and incorporated in the approach.

Attainment Watershed Approach The attainment watershed approach is a common approach for TMDL development (Brown, 2013). The approach first identifies watersheds with similar characteristics as the watershed of interest (with impairment), but at the same time, attain water quality standards. Flow conditions in the “attainment watersheds” then set the baseline or reference for TMDL control targets. This approach is useful for situations where the basis of the impairment or the pollutant of concern is directly related to the flows generated in the watershed of interest. The major assumption of the approach is that the impaired water body would meet the water quality standards or designated use if the conditions in the impaired watershed are brought to levels similar to those in the attainment watershed.

Using Hydrology as Surrogate In urban streams where multiple stressors contribute to habitat and aquatic life impairments, the identification and prioritization of individual stressors pose substantial technical and fiscal challenges to local communities. In the meantime, the natural flow regime has long been recognized as the cornerstone for determining ecosystem flow requirements, and thus it should always be mimicked for restoring activities (Poff et al. 1997, Tharme and King 1998).

While implementing the attainment watershed approach, it is critically important that the selected attainment watershed have similar hydrologic characteristics (e.g. land use, soils, slope, climate, etc.) as the watershed of interest. When the selected attainment watershed(s) has drastically different characteristics from the watershed of interest, the analysis may result in misleading conclusions (Brown, 2013).

FIG. 1 A SAMPLE FLOW DURATION CURVE (FDC)

While numerous hydrological parameters are available for describing flow variability, the selected hydrology surrogate for water quality TMDL development has to retain strong ecological and geomorphological associations on one hand, and to be simple enough for practical implementation on the other hand (Growns and Marsh, 2000). The flow duration cure (FDC) is one of the most commonly

Innovative TMDL Development Approach A surrogate TMDL development approach was proposed based on the reviews of literature. The surrogate, which is the flow duration curve, is capable of correlating a measure of biological response in the water body to pollutants and stressors, as well as to

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designated water quality standards.

Study Sites

In the proposed TMDL development approach, attainment watersheds with characteristics that are similar to those of the study watershed were first identified. Flow duration curves were then generated for both the attainment watershed and the impaired watershed, and the surrogate TMDL for the impaired watershed can be set as high, medium, and low flow rates at different exceedance levels (e.g. 5%, 50%, and 95%). The FDCs can be normalized by watershed area in order to facilitate cross‐comparisons.

The innovative approach for developing habitat and aquatic life TMDLs using hydrology as surrogate has been demonstrated at two urban watersheds in New England, USA, namely the Shawsheen Headwaters in Massachusetts and the Indian Brook watershed in Vermont. Shawsheen was the first case in which this approach was introduced through the regulatory process (MADEP, 2003), and the Indian Brook was one of the cases that used the similar approach and approved by USEPA through the regulatory process (VTDEC, 2008). Locations of the two watersheds are illustrated with reference to the New England region of the USA in Figure 2 below.

The surrogate TMDL based on the FDC is expected to provide assurance for water quality attainment for urban streams under multiple stressors in that the curves are closely related to water quality impairments in the streams. As high flow events increase in the impaired watershed, the potential for higher pollutant loadings, more sever scouring and stream bank erosion, as well as displacement of biota all increase. On the other hand, the reduction in stream base flow may also lead to loss of habitat for low flow conditions. This choice of FDC is further validated by the fact that the curves incorporate the full spectrum of flow conditions that occur over a long period of time and consist of seasonal variations.

For impaired and attainment watersheds that have limited hydrologic data to develop the surrogate TMDL, a calibrated rainfall‐runoff watershed model is often needed to generate synthetic FDCs for both watersheds. The surrogate TMDLs can then be developed for the impaired watershed by indicating the percentage of reductions or increases in relative to selected locations on the FDCs of the attainment watershed.

Shawsheen Headwaters (Hanscom) The Shawsheen River Basin is located at northwest of the Boston metropolitan area in eastern Massachusetts, with a drainage area of about 202 km2 (78 mi2). The Shawsheen River Basin has experienced substantial development over the years. A 3.2 kilometer (2 miles) river reach in the Shawsheen River headwaters (MA‐ 83‐08) is listed for “Other Habitat Alterations” impairments, with a drainage area of about 5.2 km2 (2 mi2). The impaired Shawsheen headwater subbasin is hereafter refered to as the Hanscom watershed, mainly due to the fact that this reach of the river receives storm water runoff from the nearby Hanscom Air Force Base (HAFB) and Hanscom Airport. The Hanscom watershed is highly developed with base housing, facilities, airfields, and other support facilities (e.g., school, hospital, office complexes, etc.). A comparison between the existing Hanscom land use and the land use in 1942 (the year Hansome AFB is built) is shown in Figure 3. As a result of the substantial development shown in the figure, this stretch of the Shawsheen river is significantly altered by channelization, culvertization, riparian encroachment, road crossings, and hydromodification.

100%

75%

Other Buildouts Residential Weltand

50%

Agriculture Forest

25%

0%

FIG. 2 LOCATIONS OF THE SHAWSHEEN WATERSHED IN MASSACHUSETTS, USA AND INDIAN BROOK WATERSHED IN VERMONT, USA

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Background

Current

FIG. 3 LANDUSE CHANGES AT HANSCOM WATERSHED (BACKGROUND AND CURRENT REPRESENT THE LAND USES IN 1930S AND 2000S RESPECTIVELY).

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Shawsheen watershed, the impairment could not be linked with unique stressor(s).

The substantial level of development contributes to degradation of hydrologic and water quality conditions in the Hanscom watershed. As indicated through the analysis of U.S. Geological Survey (USGS) observations at HAFB, frequent high flows (or flash flows or flood flow) and substantial low flows (or lack of base flow) became common in the Shawsheen headwaters (MADEP, 2003). Excessive concentrations of copper, zinc, and silver were also identified in the wet weather sampled collected at Hanscom watershed (Rizzo Associates, 1996). Biological assessments conducted in the watershed also found that the benthic communities were seriously impaired, as evidenced by more pollutant tolerant families and their association with organic matter and materials often found in polluted water (Rizzo Associates, 1996; MRWC, 1998,1999). The hydrologic and water quality observations confirmed that the habitat impairment is partially associated with the pollutants transported in the stream and associated with stormwater runoff.

However, the impairments could not be attributed to unique stressor(s), and this posed substantial challenge for developing traditional TMDLs for the watershed.

Stormwater Runoff and Associated Impacts Data collection and analysis results indicated that main stressors for both the Shawsheen headwater and the Indian Brook watersheds are contaminants associated with storm water runoff (e.g., sediments, metals, etc.), hydrologic modifications (excessive and insufficient stream flow rates), riparian corridor encroachment (the area and landscaping adjacent to the stream), and channel alteration. All of these stressors contributed to the impairment of aquatic life and habitat in both watersheds. While the stressors can all be linked back to the common source of stormwater runoff, the exact role and significance of each pollutant/stressor in the overall impairment, however, is difficult to quantify.

The innovative surrogate approach is used for developing comprehensive TMDLs in the Hanscom and Indian Brook watersheds. As previously discussed, attainment watersheds were first identified for each of the two watersheds. Long‐term flow duration curves were then generated for both the impaired and the attainment watersheds. Appropriate high and low flow metrics targets were then established for the impaired watershed in the development of TMDLs, using the corresponding attainment watershed metrics as reference.

Indian Brook Watershed The Indian Brook Watershed is located in the northwest of Vermont, USA (Figure 2). The stormwater impaired portion of the watershed is within Chittenden County, with a total area of 19.34 km2 (4,880 acre). Headwater for the Indian Brook watershed originates in the Town of Essex and flows south, eventually draining to Lake Champlain. The Indian Brook watershed water body is designated as cold water fish habitat pursuant to the Vermont Water Quality Standards. Land use breakdowns for the watershed include 39 percent of developed lands, 18 percent of agriculture or open lands, and 43 percent of forest or wetlands (VTDEC, 2008). A 6.4 kilometer (4 miles) stretch of the stream between the Suzie Wilson Road and the Rt. 15 are designated as impaired from excessive stormwater runoff and non‐supportive of aquatic life (VTDEC, 2008).

Results Shawsheen Headwater The attainment watershed for the Hanscom watershed is a separate headwater tributary within the Shawsheen watershed. The Elm Brook, with similar land cover, topography, and geology as Hanscom watershed, is bordering with the Hanscom watershed to the west. Upper portion of the Elm Brook is dominated by wetlands, heavy vegetation and slow moving water, and the lower portion has increasing levels of residential and commercial developments.

Biological monitoring results indicated that the Elm Brook meets Massachusetts Water Quality Standards for aquatic life (MADEP, 2003).

Biological communities are subjected to many stressors in the Indian Brook watershed. The stressors, directly or indirectly related to stormwater runoff, vary from increased pollutant load, to habitat degradation, washout of biota, and to loss of habitat due to reductions in stream base flow (VTDEC, 2008). The stressors could act either individually or cumulatively in the overall degradation of the biological community in the watershed. Similar to the situations at the

The Generalized Watershed Loading Function (GWLF) model was created and calibrated (Saravanapavan, et al., 2000, 2004) to derive long‐term flow conditions at the Hanscom and Elm Brook watersheds. The FDCs for the two watersheds are shown in Figure 4, in which the flows are expressed as the percentages to

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the median flow rate.

results indicated that the identified hydrological target would reduce the pollutant load (sediment, metals, etc.) associated with stormwater runoff by 50 percent, and pollutants were assumed to be transported from the watershed to the river by surface runoff (MADEP, 2003). The Shawsheen watershed TMDL development served as pioneering examples to many other flow‐ based surrogate TMDLs around the country (OEPA, 2012).

Indian Brook Watershed

FIG. 4 FDCS OF HANSCOM (DASHED LINE; IMPAIRED) AND ELM BROOK (SOLID LINE; ATTAINMENT) WATERSHEDS

In the development of the high and low flow metrics for the TMDL development in the Hanscom watershed, the 5 percent exceedance flow (high flow) and 95 percent exceedance flow (base flow or low flow) were chosen. A comparison of the flow metrics between the impaired Hanscom watershed and the attainment Elm Brook watershed is shown in Table 1. As shown, low and high flows from the impaired watershed are about 33 and 571 percent of the median flow rate, respectively. At the same time, the ratios are about 53 percent and 231 percent for the attainment watershed, respectively, indicating that high flows from the impaired watershed are relatively higher than the attainment watershed and the low flows are lower, which is in accord with hydrologic responses from development.

TABLE 1 FLOW METRICS FOR THE IMPAIRED HANSCOM AND THE ATTAINMENT ELM BROOK WATERSHEDS

Hydrology indicator

Hanscom (mpaired)

Elm Brook (ttainment)

95% exceedance flow (cfs/mi2) – Low Flow

0.39 cfs/mi2

0.87 cfs/mi2

50% exceedance flow (cfs/mi2) – Median Flow

1.21 cfs/mi2

1.63 cfs/mi2

5% exceedance flow (cfs/mi2) – High Flow

6.90 cfs/mi2

3.77 cfs/mi2

The comparison shown in Table 1 helped establish the target for TMDL development in the Shawsheen headwater. In order for the impaired Hanscom watershed to mimic runoff conditions in the attainment Elm Brook watershed, a 45 percent reduction of the high flow (or 3.11 cfs/mi2 for the 5 percent exceedence flow) and a 35 percent increase of the low flow (or 0.14 cfs/mi2 for the 95 percent exceedance flow) would be required. Simulation

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In order to identify the attainment watersheds for impaired watersheds, a statistical analysis was carried out for grouping impaired and attainment non‐ mountainous watersheds with similar characteristics (Foley and Bowden, 2005). In the analysis, impaired watersheds were identified by the state with degraded biotic characteristics due to stormwater runoff.

Attainment watersheds were watersheds that had a certain degree of development but currently attain the state’s biocriteria standards. The study used k‐means cluster analysis in combination with hierarchical cluster analysis to produce statistically defensible results for grouping impaired and attainment watersheds in the development of watershed‐level flow targets. A range of 12 watershed characteristics related to hydrologic runoff conditions was included in the cluster analysis (Foley and Bowden, 2005).

Through the cluster analysis, the Indian Brook watershed was grouped with several other attainment watersheds that are similar in watershed characteristics. Results from one of the attainment watersheds, the Sheldon Springs watershed, are shown here for illustration purposes. A 10‐year continuous P‐8 (Program for Predicting Polluting Particle Passage through Pits, Puddles, and Ponds) model was developed for both the Indian Brook Watershed and the Sheldon Springs watershed, and the high and low flow metrics for the two watersheds were identified and summarized in Table 2. As shown, the low flow metric is the flow exceeding 95 percent of the time, and the high flow is the flow exceeding only 0.3 percent of the time. The low and high flows are about 18 and 1012 percent of the median flow rates, respectively for the impaired Indian Brook watershed, while the ratios are about 18 and 745 percent for the attainment Sheldon Springs watershed, respectively.

The complete FDC comparison between the two watersheds is shown in Figure 5, in which the flow rates are also expressed as percentages of the median flow rates.

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TABLE 2 FLOW METRICS FOR THE IMPAIRED INDIAN BROOK AND THE

the traditional “pollutant of concern” approach and addresses adverse water quality impacts directly. This is mainly due to the fact that loads of sediment and other pollutants are a function of the amount of stormwater runoff generated from a watershed.

Additionally, the surrogate approach allows for the accounting of physical impacts to the stream channels caused by stormwater runoff. Such impacts, varying from sediment release from channel erosion to scouring from increased flows, are primary contributors to aquatic life impairments. As the surrogate TMDL sets targets for increasing low flow, aquatic lives dependent upon base flow are also addressed. Overall, the approach is in accord with the notion of sustainable water resources management, in which the maintenance of environmental flows capable of sustaining healthy river ecosystems should be viewed both as a goal and as a primary measure of success (Richter, 2009).

ATTAINMENT SHELDON SPRINGS WATERSHEDS

Hydrology indicator

Indian Brook (Impaired)

Sheldon Springs (Attainment)

95% exceedance flow (cfs/mi2) – Low Flow

0.21 cfs/mi2

0.22 cfs/mi2

50% exceedance flow (cfs/mi2) – Median Flow

1.15 cfs/mi2

1.24 cfs/mi2

0.3% exceedance flow (cfs/mi2) – High Flow

11.64 cfs/mi2

9.24 cfs/mi2

While the proposed FDC approach for TMDL development shares many similarities with the LDC approach, the two approaches are different from each other in terms of applicability. Similar to the LDC approach, the FDC approach assumes that strong correlation exists between habitat and aquatic life impairment and flow conditions (USEPA, 2007). Thus, both methods will not work when factors other than flow significantly affect the water quality conditions of a water body (Shen and Zhao, 2010). When being implemented, however, the FDC approach requires much less data than the LDC approach, which often involves extensive water quality data collection over a wide range of flow conditions. Such data are especially challenging to obtain for habitat and aquatic life impairment situations. More importantly, the FDC approach offers an unique opportunity for watersheds with impairments caused by multiple, unknown stressors to begin addressing water quality issues with reasonable confidence. This will be difficult through the LDC approach, which requires the identification of stressor(s) and the relative importance of stressor(s) to begin the analysis. In comparison, the FDC hydrology surrogate approach can bypass these prerequisites, and the analysis results can guide the implementation of practices such as imperviousness demolition, groundwater recharge, and green infrastructure, etc.

Such practices are well tested for the improvement of watershed hydrologic conditions, which eventually leads to restoration of water quality.

FIG. 5 FDCS OF INDIAN BROOK (IMPAIRED) AND SHELDON SPRING (ATTAINMENT) WATERSHEDS

Based on the comparison of FDC high and low flow metrics between the impaired Indian Brook watershed and the average runoff conditions from the group of attainment watersheds, a flow‐based TMDL was developed for the Indian Brook watershed and approved by U.S. EPA in November 2008 (VTDEC, 2008). In the TMDL, high flow from the Indian Brook watershed was set to decrease by 0.9 percent to 11.54 cfs, while the low flow target was set to increase by 0.4 percent to 0.211 cfs (VTDEC, 2008).

Discussion and Conclusions An innovative approach to develop surrogate TMDLs for urban streams under multiple stressors has been introduced in this study. The approach, based on existing knowledge regarding flow variability and corresponding stream ecological integrity, uses flow duration curve as quantifiable and practical control targets. High, medium, and low flow values on the FDC are used to set the TMDL targets. The approach was demonstrated at two locations in the New England region through the TMDL regulatory process.

In addition to filling the existing gap between urban streams with habitat and aquatic life impairments caused by multiple stressors and a practical and implementable TMDL, the surrogate TMDL bypasses

The approach presented in this study does not attempt

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to replace traditional TMDL approaches, in which stressors are often clearly identified and limits for a particular pollutant are calculated and apportioned as an overall load among point and nonpoint sources.

Instead, the approach presented here is specifically developed for impaired urban streams under multiple stressors, and more importantly, no single pollutant can be identified as the main stressor in these impairments. As a result, the TMDL targets following this approach are specified as changes in watershed runoff conditions at various flow exceedance levels, instead of the apportioned load reduction targets as obtained through traditional TMDLs. Whenever fiscally and technically possible, a continuous monitoring program should be included as an integral part of a water quality assessment scheme in the development of TMDLs, coupled with physically‐ based watershed modelling analyses. That is the traditional and reliable way to quantify the relative magnitude of different sources, their location in the watershed, their changes over time, as well as reactions to different management practices. As previously discussed, this relatively simple approach is not likely to succeed when the actual stressor(s) is not related to stormwater runoff. The approach also has not been tested for situations involving significant anthropogenic water withdrawal and release activities which are prevlant in reservoir management for water distribution and in irrigation practices in rural watersheds.

Recent Regulatory Developments Some recent developments are noteworthy relating to the surrogate approach. In January 2013, a federal court in Virginia struck down a TMDL that uses runoff volume as the surrogate for sediment in Accotink Creek watershed of Fairfax County, Virginia. While a detailed discussion of the technical, legal, and regulatory details of the case is beyond the scope of this paper, several marked differences exist between the surrogate approach used in the Accotink Creek case and the surrogate approach proposed here. First, the proposed approach here is designed for urban streams that are under multiple stressors and the relative importance of each stressor is unknown, whereas the main stressor in Accotink Creek has been clearly identified as sediment. This is a typical situation where the traditional TMDL development approach has been widely applied. Second, the proposed approach requires that the attainment watershed retains similar hydrologic characteristics as

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the impaired watershed. However, attainment and impaired watersheds in the Accotink Creek case have different soils compositions (Brown, 2013). In such situations, statistical analysis can help to identify appropriate attainment watersheds, similar to the process used for identifying the attainment watershed for Indian Brook (Foley and Bowden, 2005).

Besides the two demonstration watersheds in this study, the surrogate approach was also adopted in the Penjajawoc Streams in Maine, USA (MEDEP, 2007).

More recently, the surrogate approach was applied to the Lower Grand River watershed TMDL (OEPA, 2012) in northeast Ohio and with great success. As EPA is currently in the process of updating the stormwater program, it is expected that the surrogate approach will be adopted by more municipalities in developing practical and meaningful TMDL implementation programs.

ACKNOWLEDGMENT

The authors would like to acknowledge the following people and organizations for their support for the study: Mr. Bill Dunn, Mr. Rick Dunn, and Dr. Russ Isaac, MA DEP, Dr. Steve Silva, Mr. Eric Perkins, and Ms. Jennie Bridge, US EPA Region 1, Mr. Pete LaFlamme and Mr. Tim Clear, VT DEC, Ms. Jennifer Callahan, Vermont Agency of Trasportation, and the Merrimack River Watershed Council, Lawrence, MA. The constructive comments provided by the anonymous reviewers also helped improve the paper and are greatly appreciated.

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