Total Maximum Daily Load Process Research Articles

Phosphorus and the Chesapeake Bay: Lingering Issues and Emerging Concerns for Agriculture.

Hennig Brandt's discovery of phosphorus (P) occurred during the early European colonization of the Chesapeake Bay region. Today, P, an essential nutrient on land and water alike, is one of the principal threats to the health of the bay. Despite widespread implementation of best management practices across the Chesapeake Bay watershed following the implementation in 2010 of a total maximum daily load (TMDL) to improve the health of the bay, P load reductions across the bay's 166,000-km watershed have been uneven, and dissolved P loads have increased in a number of the bay's tributaries. As the midpoint of the 15-yr TMDL process has now passed, some of the more stubborn sources of P must now be tackled. For nonpoint agricultural sources, strategies that not only address particulate P but also mitigate dissolved P losses are essential. Lingering concerns include legacy P stored in soils and reservoir sediments, mitigation of P in artificial drainage and stormwater from hotspots and converted farmland, manure management and animal heavy use areas, and critical source areas of P in agricultural landscapes. While opportunities exist to curtail transport of all forms of P, greater attention is required toward adapting P management to new hydrologic regimes and transport pathways imposed by climate change.

Reliability-Based Water Quality Assessment with Load Resistance Factor Design: Application to TMDL.

Effective load reduction strategies rely on an accurate Total Maximum Daily Load (TMDL) calculation, which quantifies contaminant loading from various sources. There is a wide range of methods to consider uncertainties in TMDLs: from simple, conservative assumptions regarding factors that contribute to the TMDL required margin of safety (MOS), to probability-based approaches such as Monte Carlo simulations, which explicitly quantifies TMDL uncertainty. In this paper the authors adapt the Load Resistance Factor Design (LRFD), a rigorous, reliability-based framework, to water quality assessment and the TMDL process. The LFRFD replaces the lumped MOS with design factors that reflect the magnitude and distribution of uncertainty among the various contaminant loads. In addition, it produces load reduction estimates to meet management objectives with a contaminant-specific frequency-based target. The LRFD is computationally efficient and flexible in that, to compute the design factors, the procedure can utilize: measurement data, analytical solutions or model simulation results, as well as full or marginal probability distributions.

Quantification of human-associated fecal indicators reveal sewage from urban watersheds as a source of pollution to Lake Michigan

Sewage contamination of urban waterways from sewer overflows and failing infrastructure is a major environmental and public health concern. Fecal coliforms (FC) are commonly employed as fecal indicator bacteria, but do not distinguish between human and non-human sources of fecal contamination. Human Bacteroides and human Lachnospiraceae, two genetic markers for human-associated indicator bacteria, were used to identify sewage signals in two urban rivers and the estuary that drains to Lake Michigan. Grab samples were collected from the rivers throughout 2012 and 2013 and hourly samples were collected in the estuary across the hydrograph during summer 2013. Human Bacteroides and human Lachnospiraceae were highly correlated with each other in river samples (Pearson’s r = 0.86), with average concentrations at most sites elevated during wet weather. These human indicators were found during baseflow, indicating that sewage contamination is chronic in these waterways. FC are used for determining total maximum daily loads (TMDLs) in management plans; however, FC concentrations alone failed to prioritize river reaches with potential health risks. While 84% of samples with >1000 CFU/100 ml FC had sewage contamination, 52% of samples with moderate (200–1000 CFU/100 ml) and 46% of samples with low (<200 CFU/100 ml) FC levels also had evidence of human sewage. Load calculations in the in the Milwaukee estuary revealed storm-driven sewage contamination varied greatly among events and was highest during an event with a short duration of intense rain. This work demonstrates urban areas have unrecognized sewage inputs that may not be adequately prioritized for remediation by the TMDL process. Further analysis using these approaches could determine relationships between land use, storm characteristics, and other factors that drive sewage contamination in urban waterways.

Evaluation of watershed-derived mass loads to prioritize TMDL decision-making.

A total maximum daily load (TMDL) for oxygen demanding substances is being implemented in the San Joaquin River (SJR) in California (USA) due to frequently occurring low dissolved oxygen conditions. The SJR is a eutrophic river, heavily impacted by agriculture. A mass balance was developed to identify the sources of oxygen-demanding substances and nutrients to the river with the objective of providing a scientific basis for management actions needed to meet TMDL requirements. Data were collected for flow and water quality and mass loads calculated for sites within the main stem of the SJR, river inputs (tributaries), and diversions in the study area. Using a quadrant analysis, tributary flows and loads are ranked to identify targets for water quality improvement efforts. Additionally, all mass loads were summed (inputs minus diversions) and compared with observed loads at the downstream limit of the study area. The mass balance analysis identifies major contributors of mass loads and mass balance closure is assessed for each constituent. These analysis methods inform the TMDL process which includes a load allocation, and is useful for determining locations for implementation of improvement projects needed to improve the health of the river.

Arguments for inclusion of economic criteria in the total maximum daily load (TMDL) allocation phase

The idea of including economic analysis in pollution control studies is not new. There is a long tradition of research modeling least cost load allocation for water pollution control that has led to a substantial awareness that failure to take costs into account in load allocations can greatly increase the costs of water pollution control. Despite this understanding, the current load allocation approach adopted under United States Environmental Protection Agency total maximum daily load (TMDL) program allocates pollutant loads among different polluters in the absence of economic criteria. Many water quality professionals question the appropriateness of including economic criteria into an already complex and cumbersome TMDL allocation process. The question is not why we do not include economics into waste load allocations, rather the question is why we should? This paper attempts to identify and present the benefits associated with the incorporation of economic criteria into the traditional TMDL allocation process. The TMDL process is considered to be a holistic watershed approach and as such should not overlook the economic consequences of pollutant allocation alternatives. This paper attempts to focus less on the mechanism of optimizing the costs and more on the benefits derived as a by-product of doing so. The advantages of incorporating economic criteria for a bacteria load allocation in the Roses Creek watershed in the Commonwealth of Virginia are presented in this paper.

Water Quality Regulations in the Chesapeake Bay: Working to More Precisely Estimate Nutrient Loading Rates and Incentivize Best Management Practices in the Nursery and Greenhouse Industry

Restoration efforts in the Chesapeake Bay recently intensified with the 2010 introduction of federal total maximum daily load (TMDL) limits for all 92 bay watershed segments. These regulations have specific, binding consequences if any of the six states or the District of Columbia fail to meet interim goals, including loss of federal dollars for various programs and increasing regulation of point sources, if non-point source (agricultural and urban) nutrient reduction goals are not met in the watershed. As part of the effort to better understand and account for non-point sources of pollution in the watershed, a team of agricultural experts from across the bay region was recently assembled, including the nursery industry. The goal of this panel was to inform stakeholders and policymakers about the inputs and management practices used across all Bay states. To increase both the precision and accuracy of loading rate estimates, more precise information should guide future iterations of the Chesapeake Bay model. A more accurate accounting of land area by operation type (e.g., greenhouse, container, and field) is a primary issue for the nursery and greenhouse industry, because the current Chesapeake Bay model relies on USDA agricultural census data, which does not separate container and field production, which have very different nutrient and irrigation practices. Field operations also typically account for a higher percentage of production area in each state, which may skew model results. This is very important because the type of operation (field, container-nursery, or greenhouse operation) has a significant impact on plant density, types of fertilizer used, and application rates, which combine with irrigation and water management practices to affect potential nutrient runoff. It is also important to represent a variety of implemented best management practices (BMPs) in the Chesapeake Bay model such as vegetated buffer strips, sediment ponds, controlled-release fertilizer, and accurately assess how these mitigate both nutrient and sediment runoff from individual operations. There may also be opportunities for growers who have implemented BMPs such as low-phosphorus slow-release fertilizers (SRF), precision irrigation, etc., to gain additional revenue through nutrient trading. Although there are currently some questions about how nutrient trading will work, this could provide additional incentives for further implementation of BMPs by both ornamental and other agricultural growers. It is possible that the TMDL process currently being implemented throughout the Chesapeake Bay will be used as a remediation process for other impaired estuarine water bodies, which have similar water-use regulations and issues. The lessons learned about the Chesapeake Bay model in general, and for the nursery and greenhouse industry in particular, will likely provide guidance for how we can be proactive in reducing environmental impacts and protect the economic viability of ornamental growers in the future.

Hydrologic Connectivity for Highway Runoff Analysis at Watershed Scale

State transportation agencies are a potential stakeholder in the total maximum daily load process and, in some cases, NPDES stormwater permits could be issued …

Sediment Source Fingerprinting: Transforming From a Research Tool to a Management Tool1

Abstract: Information on the nature and relative contribution of different watershed sediment sources is recognized as a key requirement in the design and implementation of targeted management strategies for sediment control. A direct method of assessing sediment sources in a watershed that has attracted attention in recent years is sediment fingerprinting. The aim of this article is to describe the development of sediment fingerprinting as a research tool and to consider how the method might be transformed from a research tool to a management tool within a regulatory framework, with special reference to the United States total maximum daily load (TMDL) program. When compared with the current source assessment tools in developing sediment TMDLs, sediment fingerprinting offers considerable improvement as a tool for quantifying sources of sediment in terms of source type (e.g., channel vs. hillslope) as well as spatial location (subwatershed). While developing a conceptual framework for sediment TMDLs, we recognize sediment fingerprinting along with sediment budgeting and modeling as valuable tools in the TMDL process for developing justifiable sediment TMDLs. The discussions presented in this article may be considered as a first step toward streamlining the sediment fingerprinting approach for its wider application in a regulatory framework.

Pyrethroid and organophosphate pesticide‐associated toxicity in two coastal watersheds (California, USA)

Portions of the Santa Maria River and Oso Flaco Creek watersheds in central California, USA, are listed as impaired under section 303(d) of the Clean Water Act and require development of total maximum daily load (TMDL) allocations. These listings are for general pesticide contamination, but are largely based on historic monitoring of sediment and fish tissue samples that showed contamination by organochlorine pesticides. Recent studies have shown that toxicity in these watersheds is caused by organophosphate pesticides (water and sediment) and pyrethroid pesticides (sediment). The present study was designed to provide information on the temporal and spatial variability of toxicity associated with these pesticides to better inform the TMDL process. Ten stations were sampled in four study areas, one with urban influences, and the remaining in agriculture production areas. Water toxicity was assessed with the water flea Ceriodaphnia dubia, and sediment toxicity was assessed with the amphipod Hyalella azteca. Stations in the lower Santa Maria River had the highest incidence of toxicity, followed by stations influenced by urban inputs. Toxicity identification evaluations and chemical analysis demonstrated that the majority of the observed water toxicity was attributed to organophosphate pesticides, particularly chlorpyrifos, and that sediment toxicity was caused by mixtures of pyrethroid pesticides. The results demonstrate that both agriculture and urban land uses are contributing toxic concentrations of these pesticides to adjacent watersheds, and regional water quality regulators are now using this information to develop management objectives.

Incorporating uncertainty and objective load reduction allocation into the Total Maximum Daily Load process in Korea

This paper presents advancements to the approach for Total Maximum Daily Load (TMDL) development in Korea. The current Korean TMDL process does not directly consider uncertainty and it does not include an objective approach to allocate pollutant load reductions. This paper develops a methodology to address uncertainty by incorporating a margin of safety (MOS) based on three approaches: judgment, standard error of loading characteristics, and uncertainty and variability of loading characteristics. In addition, three pollutant load reduction allocation approaches are compared to assess their impact: equal load, equal percent, and equity standard reductions. These two proposed additions to the TMDL process in Korea are demonstrated for a BOD TMDL development in the Anyangcheon Watershed. The results confirm the importance of incorporating uncertainty into the TMDL process and clearly illustrate the significant differences in TMDL and load allocation produced when using different approaches to estimate the MOS. Further, the comparison of the three load allocation strategies highlights the potential limitations of equal load and equal percent reduction approaches. The use of technical expert and decision maker ranking of factors create an equitable and sustainable load allocation among the sub-watersheds. It can be concluded that incorporating uncertainty and providing an objective load allocation strategy are essential elements needed for future TMDL development in Korea.

Category 4b Demonstration for Point Source Impaired Waters and the Permit in Lieu of a Total Maximum Daily Load Process for the State of Missouri

Category 4b Demonstration for Point Source Impaired Waters and the Permit in Lieu of a Total Maximum Daily Load Process for the State of Missouri

Ranking methods to set restoration and remediation priorities on a watershed scale

The establishment of a total maximum daily load (TMDL) is part of management process that results in the institution of watershed-based controls of otherwise unregulated sources of pollution. In California (USA), the implementation of a TMDL is driven forward in a process where watershed stakeholders are expected to cooperate on actions needed to improve ecosystem health. In the TMDL process, methods are needed for synthesizing complex scientific data into actionable management information. Where pollutant load analysis may be misleading or perceived as unfair, non-parametric statistical methods can be applied to flow and water quality data to guide the selection of drainages for remediation. The calculation of normalized rank means (NRMs) for flow and water quality can be used to set priorities for the implementation of TMDL management actions. Drainages can be classified into one of four categories (quadrants) based on the relationship between flow and water quality NRMs. Drainages can be included or excluded from management action based on their quadrant classification. Although there are many possible alternative approaches, this "quadrant analysis" is suggested as a scientifically rigorous methods for identifying priority watersheds in the often contentious, stakeholder driven TMDL implementation process.

Spatial and Quantitative Approach to Incorporating Stakeholder Values into Total Maximum Daily Loads: Dominguez Channel Case Study

ABSTRACT The federal Clean Water Act (CWA) Section 303(d)(1)(A) requires that states conduct a biennial assessment of its waters, identify those not achieving water quality standards, establish a priority ranking for waters on the 303(d) list of impaired waters and develop and implement Total Maximum Daily Loads (TMDLs) for these waters. The Department of Energy requested LLNL develop appropriate tools to assist in improving the TMDL process. The model we developed is a stakeholder allocation model (SAM), that uses multi-attribute utility theory to quantitatively structure the preferences of the major stakeholders. We used GIS to model a significant quantity of unknown sources. The test site was the Dominquez Channel watershed in Los Angeles, California. Preliminary results show that stakeholders can have different preferences, especially in the areas of scheduling and cost of the implementation plan. The SAM model provides the ability to see how different TMDL plans rank from the perspective of each stakeholder as well as evaluate tradeoffs in selecting a plan.

Causes of water toxicity to Hyalella azteca in the New River, California, USA

The New River (CA, USA) was created in 1905 to 1907 when the Colorado River washed out diversionary works and flowed into the Salton Basin, creating the Salton Sea. Approximately 70% of the river's current flow is agricultural wastewater from the Imperial Valley. The river is contaminated with pesticides, industrial organic chemicals, metals, nutrients, bacteria, and silt. Monitoring for the State of California Surface Water Ambient Monitoring Program has indicated persistent water column toxicity to the epibenthic amphipod Hyalella azteca. Four toxicity identification evaluations (TIEs), along with chemical analyses, were performed, and the results indicated multiple and varying causes of toxicity. The first two TIEs characterized the causes of toxicity as a combination of metals and organics, but only the second sample contained enough total copper to contribute to toxicity. The third TIE used an emerging method for characterizing and identifying toxicity caused by pyrethroid pesticides. This TIE characterized organics as the cause of toxicity, and a carboxylesterase enzyme treatment further identified the cause of toxicity as pyrethroids. The final TIE used the enzyme and Phase II procedures to identify cypermethrin as the cause of toxicity. The TIE results demonstrate the evolving causes of toxicity in the New River and should assist regulators with implementing the total maximum daily load process for pesticides, particularly pyrethroids. Further research will determine if pyrethroids and other New River contaminants are having an impact on the Salton Sea.

Polychlorinated biphenyls (PCBs) in San Francisco Bay

San Francisco Bay is facing a legacy of polychlorinated biphenyls (PCBs) spread widely across the land surface of the watershed, mixed deep into the sediment of the Bay, and contaminating the Bay food web to a degree that poses health risks to humans and wildlife. In response to this persistent problem, water quality managers are establishing a PCB total maximum daily load (TMDL) and implementation plan to accelerate the recovery of the Bay from decades of PCB contamination. This article provides a review of progress made over the past 15 years in managing PCBs and understanding their sources, pathways, fate, and effects in the Bay, and highlights remaining information needs that should be addressed in the next 10 years. The phaseout of PCBs during the 1970s and the 1979 federal ban on sale and production led to gradual declines from the 1970s to the present. However, 25 years after the ban, PCB concentrations in some Bay sport fish today are still more than ten times higher than the threshold of concern for human health. Without further management action it appears that the general recovery of the Bay from PCB contamination will take many more decades. PCB concentrations in sport fish were, along with mercury, a primary cause of a consumption advisory for the Bay and the consequent classification of the Bay as an impaired water body. Several sources of information indicate that PCB concentrations in the Bay may also be high enough to adversely affect wildlife, including rare and endangered species. The greater than 90% reduction in food web contamination needed to meet the targets for protection of human health would likely also generally eliminate risks to wildlife. PCB contamination in the Bay is primarily associated with industrial areas along the shoreline and in local watersheds. Strong spatial gradients in PCB concentrations persist decades after the release of these chemicals to Bay Area waterways. Through the TMDL process, attention is being more sharply focused on the PCB sources that are controllable and contributing most to PCB impairment in the Bay. Urban runoff from local watersheds is a particularly significant pathway for PCB entry into the Bay. Significant loads also enter the Bay through Delta outflow (riverine input). Recent studies have shown that erosion of buried sediment is occurring in large regions of the Bay, posing a significant problem with respect to recovery of the Bay from PCB contamination because the sediments being eroded and remobilized are from relatively contaminated buried sediment deposits. In-Bay contaminated sites are likely also a major contributor of PCBs to the Bay food web. Dredged material disposal, wastewater effluent, and atmospheric deposition are relatively minor pathways for PCB loading to the Bay. Priority information needs at present relate to understanding the sources, magnitude of loads, and effectiveness of management options for urban runoff; the regional influence of in-Bay contaminated sites; remobilization of PCBs from buried sediment; historic and present trends; in situ degradation rates of PCBs; reliable recovery forecasts under different management scenarios; the spatial distribution of PCBs in soils and sediments; and the biological effects of PCBs in interaction with other stressors. The slow release of pollutants from the watershed and the slow response of the Bay to changes in inputs combine to make this ecosystem very slow to recover from pollution of the watershed. The history of PCB contamination in the Bay underscores the importance of preventing persistent, particle-associated pollutants from entering this sensitive ecosystem.

Approaches to TMDL planning and implementation and policy tools for implementation to achieve water quality standards.

Abstract This review is based primarily on information gathered in the TMDL Knowledgebase Clearinghouse developed by the Center for Total Maximum Daily Load (TMDL) and Watershed Studies at Virginia Tech. The TMDL programme has received renewed attention since the US Environmental Protection Agency (EPA) published revised guidelines in 1991. Under the TMDL programme, pollutant-specific TMDLs are developed for impaired waters. A TMDL specifies the pollutant load reductions needed to achieve water quality standards. The magnitude and distribution of the load reductions among pollution sources can have implications for watershed stakeholders. Implementing the TMDL load reductions is the responsibility of the individual US states. Adaptive implementation occurs when limited corrective actions designed to improve water quality are implemented using available information and data; and as information and data availability improve implementation adjustments are made, i.e. 'learn while doing'. While several policy tools are available to ensure that corrective actions are implemented, economic incentives are most often used, particularly for non-point source pollution sources. Typically, successful TMDL implementation depends on careful planning and stakeholder engagement. However, meaningful stakeholder engagement requires that stakeholders be empowered to make decisions about outcomes relevant to their goals, which may be problematic given the narrow way the TMDL process is currently enacted. Planning using watershed-scale models to assess plausible load reduction scenarios is most likely to produce credible TMDL pollutant load estimates. Models should be coupled with adaptive implementation so that pollutant load estimates can be revised as better information becomes available. TMDL implementation plans should consider both barriers to and opportunities for implementation.

Carga máxima de fósforo admissível ao reservatório Piraquara II, uma aplicação do processo TMDL

Para implementação e operacionalização da política brasileira de recursos hídricos, é imprescindível o uso de ferramentas de planejamento que considerem o efeito de todas as atividades ou processos que causam ou contribuem para a degradação da qualidade de um corpo d'água. Neste sentido, aplicou-se o processo TMDL (total maximum daily load), desenvolvido pela Agência de Proteção Ambiental dos Estados Unidos (EPA), para o P, na área de drenagem de contribuição ao futuro reservatório Piraquara II, bacia hidrográfica do rio Piraquara, Paraná. O processo TMDL determina a quantidade máxima de cargas de um poluente que um corpo d'água pode receber sem violar os padrões estabelecidos de qualidade da água e aloca cargas deste poluente entre fontes de poluição pontuais e difusas. No presente estudo, utilizou-se o método TMDL, com o objetivo de demonstrar ser ele uma ferramenta útil no processo de gestão dos recursos hídricos. Simularam-se cenários de uso do solo, por meio de modelagem matemática, até obter-se uma concentração de P total no reservatório abaixo da faixa limite para ocorrência de eutrofização, de 0,025 a 0,10 mg L-1, estabelecida no estudo. Realizou-se uma simulação de uso atual do solo, visando prever a condição inicial de qualidade da água no corpo d'água, na qual a concentração de P total no reservatório resultante não atendeu ao padrão estabelecido. Procedeu-se a uma segunda simulação com adoção das medidas de controle, recomposição de mata ciliar e plantio direto, para reduzir a exportação de carga de P total da bacia. Obteve-se uma melhoria na qualidade da água do reservatório, indicando que as medidas adotadas foram suficientes para atingir o padrão estabelecido, o que demonstra a aplicabilidade do método.

UNCERTAINTY IN TMDL MODELS

Although the U.S. Congress established the Total Maximum Daily Load (TMDL) program in the original Clean Water Act of 1972, Section 303(d), it did not receive attention until the 1990s. Currently, two methods are available for tracking pollution in the environment and assessing the effectiveness of the TMDL process on improving the quality of impaired water bodies: field monitoring and mathematical/computer modeling. Field monitoring may be the most appropriate method, but its use is limited due to high costs and extreme spatial and temporal ecosystem variability. Mathematical models provide an alternative to field monitoring that can potentially save time, reduce cost, and minimize the need for testing management alternatives. However, the uncertainty of the model results is a major concern. Uncertainty is defined as the estimated amount by which an observed or calculated value may depart from the true value, and it has important policy, regulatory, and management implications. The source and magnitude of uncertainty and its impact on TMDL assessment has not been studied in depth. This article describes the collective experience of scientists and engineers in the assessment of uncertainty associated with TMDL models. It reviews sources of uncertainty (e.g., input variability, model algorithms, model calibration data, and scale), methods of uncertainty evaluation (e.g., first-order approximation, mean value first-order reliability method, Monte Carlo, Latin hypercube sampling with constrained Monte Carlo, and generalized likelihood uncertainty estimation), and strategies for communicating uncertainty in TMDL models to users. Four case studies are presented to highlight uncertainty quantification in TMDL models. Results indicate that uncertainty in TMDL models is a real issue and should be taken into consideration not only during the TMDL assessment phase, but also in the design of BMPs during the TMDL implementation phase. First-order error (FOE) analysis and Monte Carlo simulation (MCS) or any modified versions of these two basic methods may be used to assess uncertainty. This collective study concludes that a more scientific method to account for uncertainty would be to develop uncertainty probability distribution functions and transfer such uncertainties to TMDL load allocation through the margin of safety component, which is selected arbitrarily at the present time. It is proposed that explicit quantification of uncertainty be made an integral part of the TMDL process. This will benefit private industry, the scientific community, regulatory agencies, and action agencies involved with TMDL development and implementation.

Improving the TMDL Process Using Watershed Risk Assessment Principles

Ecological risk assessment (ERA) evaluates potential causal relationships between multiple sources and stressors and impacts on valued ecosystem components. ERAs applied at the watershed scale have many similarities to the place-based analyses that are undertaken to develop Total Maximum Daily Loads (TMDLs), in which linkages are established between stressors, sources, and water quality standards, including support of designated uses. TMDLs focus on achieving water quality standards associated with attainment of designated uses. In attempting to attain the water quality standard, many TMDLs focus on the stressor of concern rather than the ecological endpoint or indicators of the designated use that the standard is meant to protect. A watershed ecological risk assessment (WERA), at least in theory, examines effects of most likely stressors, as well as their probable sources in the watershed, to prioritize management options that will most likely result in meeting environmental goals or uses. Useful WERA principles that can be applied to TMDL development include: development and use of comprehensive conceptual models in the Problem Identification step of TMDLs; use of a transparent process for selecting Numeric Targets for TMDLs based on assessment endpoints derived from the management goal or designated use under consideration; analysis of co-occurring stressors likely to cause beneficial use impairment based on the conceptual model; use of explicit uncertainty analyses in the Linkage Analysis step of TMDL development; and frequent stakeholder interactions throughout the process. WERA principles are currently most applicable to those TMDLs in which there is no numeric standard and, therefore, indicators and targets need to be developed, such as many nutrient or sediment TMDLs. WERA methods can also be useful in determining TMDL targets in situations where simply targeting the water quality standard may re-attain the numeric criterion but not the broader designated use. Better incorporation of problem formulation principles from WERA into the TMDL development process would be helpful in improving the scientific rigor of TMDLs.

A Predictive Approach to Nutrient Criteria

Violation of a water quality standard triggers the need for a total maximum daily load (TMDL); this should result in actions that improve water quality, but sometimes at significant cost. If the standard is well-conceived, a designated-use statement characterizes societal values, and a criterion provides a measurable surrogate for designated use. This latter provision means that scientists measure the criterion and view violations of the criterion as equivalent to noncompliance with the designated use. However, if a criterion is not a good indicator of designated use, it is apt to result in misallocation of the limited resources for water quality improvement through the TMDL process. This concern provides the basis for our assessment of the national nutrient criteria strategy recently proposed by the U.S. EPA. We acquired data sets for four case studies (Lake Washington, Neuse River Estuary, San Francisco Bay, and Lake Mendota) and then used expert elicitation to quantify designated-use attainment for each case. Applying structural equation modeling, we identified good water quality criteria as the best predictors of the designated use elicited response variable. Further, we used the model to relate the level (concentration) of each criterion to the probability of compliance with the designated use; this provides decision-makers with an estimate of risk associated with the criterion level, facilitating the selection of appropriate water quality criteria.