NE1545: Onsite Wastewater Treatment Systems: Assessing the Impact of Climate Variability and Climate Change

(Multistate Research Project)

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The Need

Onsite wastewater treatment systems (OWTS) serve approximately 25% of households in the United States and are the technology of choice in rural and suburban areas where population density and cost preclude the use of centralized sewer collection and treatment systems. In unsewered watersheds they are the sole means of wastewater treatment. Onsite wastewater treatment systems are an integral part of the water infrastructure throughout the country, which are expected to protect ground and surface water quality from inputs of carbon, nutrients, pathogens, and pharmaceutical & personal care compounds, and to do so under a wide range of environmental conditions with little intervention.

Meeting these expectations requires an understanding of the processes on which OWTS rely to treat wastewater in order to engineer systems that function effectively under a wide variety of conditions. This is particularly challenging for OWTS, which rely on hydraulic, hydrologic, physical, chemical and biological processes – and their interactions – to treat wastewater. Despite their ubiquity and importance as part of the nation’s water infrastructure, our understanding of these processes in OWTS lags behind that for centralized sewage treatment systems.

The systematic study of OWTS has evolved considerably over the past five decades, leading to improvements in understanding of how contaminant removal takes place within components in the treatment train and the receiving soil. This has led to more effective contaminant removal from changes in system design, improved understanding of the biogeochemical processes that remove contaminants, improved selection of soils receiving wastewater, and better placement of systems within watersheds to maximize treatment and minimize impact. These improvements have come about, in large measure, from the efforts of scientists, engineers and outreach professionals in Land Grant and private universities across the U.S., funded by federal, state and local agencies, and through collaborations with regulators and private industry.

As many of the contaminant removal concerns associated with OWTS are being addressed, a number of new challenges have developed in different parts of the country, such as more stringent nutrient reduction regulations, and concerns with removal of pharmaceuticals and personal care products present in wastewater. In addition, a changing climate presents a continental-scale challenge to OWTS. Soil-based wastewater treatment systems are regulated, designed and built based on assumptions about the volume of wastewater applied, the magnitude and distribution of past precipitation events, the historical range of variations in depth to water table, and soil temperature over the long-term (decades). These assumptions are no longer valid in many parts of the country because of climate variability and climate change (CV/C). Climate variability refers to the variation from the long-term (30-year) average weather conditions. Climate change refers to a long-term (decades), continuous change in average weather conditions, such as temperature, or the range of weather events, such as more severe and frequent extreme storms (Michigan Sea Grant, 2015). Climate change is one of the drivers of climate variability, along with events such as El Nino and La Nina and volcanic eruptions.

Climate change and climate variability have altered, and will continue to alter, the temporal and spatial patterns of precipitation and temperature, with attendant consequences as sea levels rise and changes in groundwater levels develop. These changes will affect populations that rely on OWTS, through effects on soil moisture dynamics, surface and groundwater hydrology, changes in water use patterns and associated changes in wastewater composition and volume, and temperature effects on processes within treatment trains and soil-based treatment. Regulatory decision-makers that set codes and policies and other stakeholders need to understand the consequences of these changes and the options available to mitigate, adapt, and plan for climate change and its effects on OWTS. As in the past, they rely on scientists, engineers and outreach professionals to carry out the research and provide them the necessary information in an effective and timely manner.

Importance of the Work

Scientists and engineers have developed a reasonable understanding of many of the processes that underlie the functioning of soil-based wastewater treatment over the past 50 years, with work conducted recently by NE1045 members helping to provide deeper understanding. This understanding continues to develop, as does our understanding of the challenges that CV/C pose to OWTS function and longevity. These changes will affect our assumptions about the magnitude and frequency of precipitation events, inputs of freshwater to OWTS soil treatment areas (STA), depth to groundwater, and temporal variability of temperature at diurnal seasonal and decadal scales.

As a “green technology” (Lindbo, 2015), onsite wastewater treatment systems are relied upon by the majority of rural and a large proportion of suburban populations to also help protect public health and sensitive ecosystems. We do not know exactly how OWTS will be affected by climate change, but we can be certain that they will be affected. They rely on an intricate set of hydrologic, biogeochemical and physical processes to renovate wastewater. These are controlled, directly and indirectly, by precipitation, temperature, and depth to groundwater. Climate change will affect the quantity and quality of wastewater inputs as a result of changes in precipitation patterns (e.g. prolonged droughts or wetter conditions) as well as soil moisture dynamics and depth to saturation, from changes in precipitation patterns and associated fluctuations in water table levels, as well as sea level rise in coastal areas. Although these factors are likely to vary in magnitude geographically, climate change is expected to impact most the U.S. population, both in inland and coastal areas.

We are already familiar with the effects of malfunctioning and poorly functioning OWTS from previous experiences: contamination of ground and surface waters with human pathogens leading to the spread of enteric diseases, as well as increased inputs of N and P to aquatic ecosystems, resulting in eutrophication, anoxia and ecosystem collapse. In most instances, incidents of malfunctioning OWTS have had a modest impact on public health and ecosystem functioning, because they are generally limited in geographic scope to one or a few systems. Because climate change is experienced at regional and continental scales, the number of malfunctioning systems, and the magnitude of their impact, is expected to be at the regional and continental scales, affecting a much larger portion of the population, as well as regional aquifers and surface aquatic ecosystems. Ecosystem effects are expected to linger for decades, particularly for nutrients like P, for which removal pathways are physical, and for N - which relies on microbial processes, with severe constraints, for removal from ecosystems.

Because drinking water sources for large urban areas are often found in rural landscapes where OWTS are common, the impact will not be limited to rural areas alone. This is particularly true with respect to increased inputs of organic C and nutrients to surface and groundwater reservoirs from OWTS, which interfere with water disinfection processes and result in the production of trihalomethanes, known human carcinogens.

Letters from four regulatory agencies in the Northeast (MA, MD, RI, VT), speaking to the importance of this research, are included in the Attachments section.

The Technical Feasibility of the Research

The proposed research is technically feasible. The tools and techniques necessary to carry out the research have been developed, and include cutting-edge approaches from a variety of science and engineering fields, such as advanced computer modeling of hydrological and biogeochemical processes at a range of spatial and temporal scales, the use of stable isotopes to identify nutrient transformations, and application of the tools of molecular genetics to identify the microorganisms responsible for both water quality degradation and improvement. The research efforts of biological and environmental engineers, pedologists, hydrologists, soil physicists, soil microbiologist, computer modelers, and extension and outreach professionals at state and private universities have led to a better understanding of the challenges presented by climate change and the potential solutions.

Advantages for Doing the Work as a Multistate Effort

Addressing the research and outreach challenges presented by climate change requires the expertise of a broad spectrum of scientists, engineers and outreach professionals. The necessary breadth of expertise and perspectives is already found in Land Grant and private universities across the U.S. Researchers throughout the country are currently working on various aspects of the science and engineering of OWTS and the challenges presented by climate change. In some instances, the results of this research are broadly applicable, as is the case with studies focusing on fundamental processes. In other instances the research focuses on addressing problems that are local or regional in nature as a result of unique geological, geographical or regulatory issues. Similarly, outreach professionals develop materials that are tailored to national, regional and local issues, depending on the circumstances. In order to carry out and disseminate research that is responsive to the broad range of problems and constituencies in the U.S., the proposed work needs to be a comprehensive and collaborative effort done at a multistate level.

The Likely Impacts from Successfully Completing the Work

We anticipate that completion of the proposed work will lead to evidence-based solutions to OWTS problems associated with climate change. Specifically, we expect that results from examination of fundamental physical, chemical and biological processes – and their response to changes in hydrologic regime and temperature – will, in combination with studies focusing on local and regional aspects of OWTS and climate change, provide solutions to the challenges of climate change at appropriate spatial and temporal scales. Furthermore, we anticipate that these solutions will be shared among OWTS outreach professionals and made available to stakeholders in an effective and timely manner so that the expected impacts of climate change on OWTS – and associated large-scale public health and ecosystem effects – can be ameliorated or averted.

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