Concentrating on dissolved oxygen

ONE of the major environmental issues associated with hydroelectric power production is the effect of project operations on water quality, particularly dissolved oxygen (DO) concentrations in the water released from the dam.

Low DO levels in the discharges from hydroelectric reservoirs most often occur as a result of seasonal warming and the consequent temperature stratification of impounded waters. During the summer, this natural process can divide the reservoir into distinct vertical strata: a warm, well-mixed upper layer (epilimnion) overlying a cooler, relatively stagnant lower layer (hypolimnion). Plant and animal respiration, decomposition of organic matter and chemical reactions can all act to progressively remove DO from bottom waters. Decomposition of sulphur and nitrogen compounds in the absence of DO may result in the buildup of toxic hydrogen sulfide and ammonia.

Low DO in the hypolimnia of hydroelectric reservoirs may be caused by the breakdown of materials in the reservoir (flooded vegetation), or by the inflow of water rich in oxygen-consuming materials, derived from the upstream watershed and independent of reservoir characteristics. Regardless of the specific cause of low DO, the release of deoxygenated water through deep turbine inlets will degrade aesthetic qualities, communities of aquatic organisms, and waste assimilative capacity in the river downstream from the powerhouse.

There are numerous structural, operational and regulatory techniques that a hydro power operator can use to resolve low DO. For example, adding air at the turbine, injecting air or oxygen in the forebay and tailrace weir aeration have proven effective at particular sites. Thorough reviews of such structural options for improving DO below dams are available (epri 1990, 2002).

Levels of DO can also be increased through modifications in dam operations, including such techniques as fluctuating the timing and duration of flow releases, spilling or sluicing water, and increasing minimum flows.

Even with structural or operational changes, DO concentrations below dams may not comply with regulatory requirements, and further modifications can be costly. This paper addresses the use of alternative regulatory strategies that are being explored in the US to address DO problems. These strategies emphasise flexibility, increased collaboration and shared responsibility among all water users, and market-based, economic incentives. Many of these options appear to have analogies in European Union (EU) water quality legislation. Details of the application of these strategies in the US can be found in Regulatory Approaches for Addressing Dissolved Oxygen Concerns at Hydropower Facilities, DOE/ID-11071 (available at http://hydropower.id.doe.gov/).

Regulatory approaches

The attention of the regulatory agencies and public most often focuses on a facility's compliance with numerical water quality standards (eg, maintenance of a DO concentration of 6mg/L or greater at prescribed times in the discharged water) and impacts to downstream fisheries. Water quality regulation in the US is moving away from such 'one-standard-fits-all' approaches toward more comprehensive, watershed-wide strategies with greater site-specific flexibility. Some regulatory options for addressing DO issues at hydro power facilities include the use of site-specific water quality standards; biocriteria; watershed-based strategies; and watershed-based trading. The following sections describe alternative approaches to conventional water quality regulation that will require additional information and negotiation. The key assumption for all of these approaches is that the current regulatory limits may not be an effective measure of water quality impacts (or lack of impacts). The hydro power facility may be able to provide a convincing argument that a healthy aquatic community could be maintained at lower numerical limits, or, if impacts are occurring, that the costs of enhancing DO levels in the river should be shared with other water users.

Site specific water quality standards

The US Clean Water Act (CWA) prohibits the discharge of pollutants into most waterways of the US without a permit issued by the Environmental Protection Agency (EPA) or the state water quality permitting agency. A discharge permit provides conditions and establishes allowable levels for the discharge of pollutants to surface waters. In the case of DO, the permit specifies minimum concentrations needed to protect water quality and other instream uses.

For many years, a single minimum allowable DO concentration of 5mg/L was the established criterion, deemed adequate to protect the diversity of aquatic life in fresh waters. Later EPA established new numerical criteria that included various mean and minimum values for cold water and warm water systems. In addition, separate criteria were established to protect early life stages of fish and for inter-gravel water in gravel spawning areas. Depending on the test method, life stage, and water temperature, DO criteria can be as low as 3mg/L (one-day minimum for warm-water 'adult' life stages) or as high as 9.5mg/L (seven-day mean for inter-gravel water to protect early life stages of coldwater fish).

There are cases of healthy aquatic communities and successful fisheries below dams despite occasional noncompliance with DO criteria. A potential approach to compliance for these facilities is to document that the aquatic community downstream is not affected by occasional DO noncompliances (through the application of a bioenergetics model or some other bioassessment) and to negotiate less stringent limits or allowable frequencies of noncompliance.

Hydro power operators with DO problems should contact the appropriate water quality regulatory authority to discuss site-specific standards.

Biocriteria and bioassessments

The objective of the CWA is to restore the physical, chemical, and biological integrity of the nation's waters. During the last two decades, increasing emphasis has been placed on measuring the biological status of surface waters as a supplement to numerical criteria that are based on chemical water quality. The ability of a water body to sustain a balanced, integrated, adaptive assemblage of aquatic organisms is one of the best overall indications that the water body is suitable for other uses.

It may be appropriate to negotiate the replacement of numerical limits for DO with biocriteria (defined by the EPA as numeric or narrative expressions that describe the reference biological integrity of aquatic communities inhabiting waters of a designated use). Examples of biocriteria include measures of species diversity or abundance, fish growth and mortality, and instream habitat measures. Bioassessments are evaluations of the biological condition of a water body that use biological surveys and other direct measurements of the resident biota.

Although chemical measurements provide information about a specific source of low-DO water, a watershed-based biomonitoring programme can provide a better understanding of actual effects of hydro power discharges on the biological integrity of receiving waters. Biological assessments can integrate impacts of all sources over space and time (biota are exposed to all upstream sources over their lifetime), in contrast to chemical measures that may reflect a grab sample taken at a single location and time. Often, biological assessments are based on comparing the fish or aquatic macroinvertebrate communities in the potentially affected river reach (eg dam tailwaters) with the aquatic communities in nearby, similar sized reference sites. If the communities in the tailwaters are not significantly different from those in the reference sites then DO non-compliance are not adversely affecting the designated uses of the water.

While the value of using the aquatic community to reflect the long-term effects of water quality is clear, there are several constraints associated with this approach. For example, there may be substantial year-to-year variation in biological measures (eg growth rate) that are not related to water quality and may overshadow the effects of reservoir discharges. In addition, it can be difficult to find similar-sized, unimpaired reference sites.

The costs of this approach can be high; biological measurements can be expensive, and data analysis and interpretation can require a high level of expertise. Nonetheless, the Oak Ridge Biological Monitoring and Abatement Program (BMAP) has successfully used a biocriteria/bioassessment approach since the mid-1980s to evaluate the condition of receiving streams on the Oak Ridge reservation.

The EU approach

Water quality is one of the most comprehensively regulated areas of EU environmental legislation. In order to integrate the varying policies and regulations of the member countries into a single piece of framework legislation, a Water Framework Directive aims to expand the scope of water protection to all surface and groundwaters; achieving 'good' status for all waters by 2015; basing water management on river basins, rather than on political or administrative boundaries; using a 'combined approach' of emission limit values and water quality standards; and incorporating overall water costs in the price of water, reinforcing the 'polluter pays' principle and confronting users with the real costs of providing water.

Two goals of the WFD are of particular relevance to the issue of DO concentrations in water discharged from hydro power facilities: the determination of status of the surface waters and the desire to employ a 'combined approach' to address water pollution.

The WFD requires that all surface waters be categorised as either 'high', 'good' or 'fair' in terms of their ecological status, and that all waters be brought to at least a 'good' status. The assessment will be based on the structure and function of ecological systems, rather than just chemical contamination. Surface waters that differ only slightly from what would be expected under conditions of minimal anthropogenic impact are considered to have high ecological status. The definitions for ecological status are based on changes in structural and functional characteristics of the aquatic ecosystems, compared to 'natural' reference sites.

Some of these ecological parameters are difficult to express quantitatively and to categorise, and the significance of deviations from the reference conditions are difficult to judge. Nonetheless, such an approach recognises the limitations of relying solely on chemical water quality standards to determine the status of surface waters. Determining the ecological status takes into account those surface waters that do not meet chemical water quality standards yet could still support a balanced aquatic ecosystem. Conversely, waters that meet standards for chemical contaminants may still be impaired because of habitat degradation, undetected pulses of contaminants, or the cumulative effects of low levels of chemical contaminants. Hydro power facilities could be evaluated on whether or not they support a normative aquatic ecosystem in the tailwaters, rather than strictly on the basis of compliance with DO standards.

The WFD recognises that there are two general approaches to regulating water quality. The water quality objective (WQO) approach defines the minimum quality requirements of a water body in order to limit the cumulative impacts of multiple effluents. On the other hand, the emission limits value (ELV) approach focuses on the maximum allowed quantities of pollutants discharged from a particular source into the aquatic environment. The WFD notes that an approach which combines WQO and ELV is needed; they will reinforce each other and in any particular situation, the more rigorous approach will be applied. All existing technology-driven source-based controls must be implemented as a first step (ELV approach). But over and above this, the requirement for good ecological status may call for additional water pollution control measures (WQO approach).

While the WFD's combined approach will help ensure the maintenance of good water quality, it does not support tradeoffs, and it lacks the flexibility of watershed-based strategies now being pursued elsewhere.

Details of the implementation of the WFD are still being worked out on both an EU and country-by-country basis, so the effects of this new legislation on hydro power production in Europe are not yet known. The first required activity is that EU Member States identify and assign water bodies to river basin districts based on hydrological catchments.

Scotland sees the new directive as an opportunity to improve the regulation of water abstractions (withdrawals), including those from approximately 1000 hydro power projects, many of which divert all the water during low river flows. The quality of water discharged from reservoirs will be judged not only by the level of chemical parameters but also by the status of the aquatic ecosystem downstream from the dam. Bringing the ecological status of surface waters up to the 'good' category may require the provision of fish passage and minimum flow releases at hydroelectric power plants. Exceptions to the requirement to achieve good ecological status are allowed for some essential activities (eg flood protection). Power generation is subject to three tests before the requirement is relaxed: alternatives are technically impossible, are prohibitively expensive, or would produce a worse overall environmental result.

Watershed-Based Strategies

Watershed management has been found to be a cost-effective means for improving tailwater quality.

Nutrient and organic material sources in the upstream watershed can be a major factor contributing to low DO concentrations in reservoir hypolimnia and below dams. Consequently, actions in the watershed above the reservoir, such as land use planning, erosion control, groundwater protection, and animal and septic waste control, can be effective ways to improve both DO levels and overall water quality.

An example of a watershed-based strategy is the total maximum daily load (TMDL) programme, which is a US Environmental Protection Agency regulatory framework by which states identify polluted waters, determine the sources of pollution, and design basin-wide cleanup plans. The TMDL is the maximum amount of a pollutant that a water body can receive and still meet water quality standards ­ the sum of all pollutant contributions from point-source discharges, nonpoint sources, plus a safety factor (called the margin of Safety, MOS).

The role of hydro power operations in this process is still not well defined. Key elements are a shared approach to water quality problems and local/regional flexibility in how water quality goals are achieved. Applying the TMDL approach involves mathematical modelling, to identify pollutant sources, evaluate loadings, examine impacts on receiving waters, quantify loading capacity, evaluate the linkage between loading and response, and allocate the pollutant loading in a way that ensures achievement of water quality standards. The TMDL approach in the US continues to evolve; the existing regulations under which the TMDL programme now operates, as well as the status of future changes in the regulations, may be found at http://www.epa.gov/owow/tmdl/.

A variation of the watershed management approach is watershed-based trading. Watershed-based trading also involves an evaluation of pollution sources in a watershed but costs and market incentives are more formally considered, and a procedure for trading pollution credits is established in an effort to achieve overall watershed-wide clean water goals. In the case of hydro power, an industry or city might pay (or obtain credits from) hydro facilities to improve DO and water quality below dams (using aeration, etc), at a potentially lower cost than remediation options at the industrial point source.

An exploratory analysis of the South Fork Holston river in Tennessee provides an example of how point/nonpoint-source pollutant trading within a watershed might be implemented. In this case, an industrialised stretch of the river downstream from Fort Patrick Henry dam did not meet water quality standards, even after the nearby city and local industries installed new wastewater treatment facilities and met their technology-based treatment and permit requirements. Although several hundreds of millions of dollars had been invested in waste treatment facilities in the 1970s, DO levels in the South Fork Holston river dropped to 2mg/L under low flow conditions.

A number of options for improving DO conditions in the river were considered, including advanced waste treatment for the dischargers, turbine aeration at Fort Patrick Henry dam, various levels of flow augmentation at the dam, and instream aeration. The analysis indicated that DO standards of 5mg/L in the South Fork Holston river could not be attained even with additional advanced effluent treatments that were considered by the industrial and municipal dischargers. Further, the costs of these treatments exceeded those for river management options at the dam by at least two orders of magnitude. A situation like this presents the point source dischargers with a clear opportunity to reduce their waste treatment costs by helping to defray the electric utility's cost of aeration and flow augmentation. But by including a hydro power project in a watershed-based trading programme, it may be possible to secure a new source of revenue for the hydro project; maximise water resource benefits when power generation has less value; improve water quality in 'water quality limited' stream segments; provide for continued economic growth where river assimilative capacity is currently limited; and enhance river habitats.

Options for addressing DO

The use of nonstructural approaches is one of the options for addressing DO concerns at hydro power facilities.

Structural modifications may be the most effective option, especially in cases where there are severe DO problems, but costs are often high. Modifying operational procedures can also improve DO without the need for structural changes. However, in cases where operational changes involve spilling water or increasing generation during times of the day when power is less valuable, the operational approach to DO enhancement can also be costly.

The use of more flexible regulatory approaches to the DO issue (eg, developing site-specific water quality standards) is an option available to hydro power facilities. Bioassessments are being increasingly recognised as the most appropriate and direct means for determining whether biological integrity of the receiving waters is maintained. Bioassessments not only ensure that discharge limits in the water quality permits are having the desired effects, they can also be used to determine whether healthy aquatic communities can be supported in the tailwaters despite occasional non-compliance with chemical (numerical) water quality standards. If water quality problems in the hydro power reservoir releases can ultimately be attributed to sources within the watershed, it may be preferable first to reduce these loads in the watershed or for dischargers in the watershed to assist in the implementation of water quality improvements in the reservoir.

The most effective strategy for addressing a DO problem must be site-specific. Before making costly structural or operational changes at hydro power facilities, a thorough assessment of the specific DO problem should be conducted. As much as possible, this assessment should quantify sources of water to the reservoir that contain high concentrations of nutrients and organic materials. It is possible that other point and nonpoint-source dischargers are having difficulty complying with water quality regulations, and they may find it most economical to help defray to costs of DO-enhancing modifications by the hydro power project. A combination of mitigation techniques, including structural, operational, and regulatory approaches, may be needed.