Date of Award

2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Biological and Environmental Sciences

First Advisor

José A. Amador

Abstract

Septic systems, or onsite wastewater treatment systems (OWTS), are a common means of treating wastewater across the world in areas where centralized wastewater treatment is not a feasible option. In the US, one in five households are served by septic systems, with a quarter to over half of New England households relying on this technology to treat wastewater. A well-functioning septic system protects public health by reducing pathogen loading to ground and surface waters, and attenuating nutrients and pollutants found in wastewater before it is recharged to local groundwater. However, not all wastewater constituents are removed during these processes, and thus OWTS can serve as a source of nutrients and other pollutants. Poorly designed systems whose components are situated without sufficient vertical separation distance to the groundwater table increase the risk of nutrient and pathogen pollution to local ground and surface water resources, with implications for both human and environmental health. Onsite wastewater treatment systems in general can also be sources of greenhouse gases, including carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) — all by-products of microbial transformations of wastewater constituents.

I examined historic and current dynamics of coastal groundwater tables along the southern Rhode Island (RI) shore, in an effort to determine whether near-shore OWTS — which are ubiquitous in these coastal communities — are at risk for compromised separation distance. Inadequate separation distance below OWTS components among the ~8,000 OWTS located within 1 km of coastal water bodies in this area puts these communities at risk for pollution of groundwater and sensitive coastal ecosystems, both of which are valuable resources to these communities. To understand historic groundwater table dynamics, I used depth to groundwater table data submitted to the RI Dept. of Environmental Management — the agency responsible for OWTS permitting — as part of permit applications for individual properties. Based on these data, groundwater tables along the southern Rhode Island coast have been rising at a rate of 14 mm/year since 1964. Areas where potable water is imported and recharged to local groundwater have higher rates of rising groundwater tables, up to 17 mm/year. Precipitation, human wastewater inputs, and sea level rise are the major factors elevating coastal groundwater tables, whereas evapotranspiration, discharge of groundwater to coastal water bodies, and drinking water extraction are the major components lowering groundwater tables. As residents in the coastal areas continue to increase groundwater recharge (e.g. via OWTS) while importing potable water (reducing local groundwater extraction), these observed rates of groundwater table rise are likely to continue or become exacerbated, especially in light of climate change, marked by more frequent and intense precipitation events, as well as sea level rise. As groundwater tables in southern RI’s coastal communities rise, onsite wastewater treatment at the ~1,500 to 2,000 properties within 200 m of a tidal water body will become increasingly compromised, posing serious risks of nutrient and pathogen pollution to both ground and surface water resources.

I also assessed how present-day groundwater tables affect OWTS installed along the southern RI coast. Data from long-term shallow groundwater monitoring wells and ground-penetrating radar surveys of 10 drainfields in the southern RI coastal zone were used to determine whether septic system drainfields have adequate separation distance from the water table. My results indicate that only 20% of tested systems are not impaired by elevated groundwater tables, while 40% of systems experience compromised separation distance at least 50% of the time. Surprisingly, 30% of systems in this study do not meet separation distance requirements at any time of the year. Neither age of system nor a system’s landscape position relative to a tidal water body were correlated with compromised separation distance. The compromised separation distances may be a result of inaccurate methods — specified by the regulations — to determine the height of the seasonal high water table. Improved methods of determining seasonal high water table elevations include (1) creating a network of long-term groundwater monitoring wells in the near-shore area, to which individual measurements during the wet season at different properties could be referenced; (2) using inexpensive indicator of reduction in soils (IRIS) tubes in addition to soil profile descriptions or individual well measurements at each site; and/or (3) increasing the required separation distance between OWTS components and the seasonal high water table to account for inaccuracies of the current approach, as well as anticipated long-term rises in groundwater table elevation.

The results of these two studies suggest that, as water inputs in southern RI continue to change in the coming decades, rising groundwater tables will reduce OWTS functionality by wide-spread reduction in separation distance, which appears to be occurring already at a fast rate. A network of long-term groundwater monitoring wells in the near-shore area across the southern RI coast would ensure that regulatory and municipal officials, as well as OWTS designers, have an accurate understanding of both long-term and short-term groundwater table dynamics in the region, and provide a reference to which individual measurements at private properties could be compared. A simpler, less data-driven solution would be to increase the required separation distance to the groundwater table, to account for the fact that current methods are inaccurate, and incorporate a buffer of sorts for the continued rise in groundwater tables along the coast. Failure to enact these proposed changes — or similar ones — in the methods of determining the groundwater table elevation at time of system design and installation may threaten coastal drinking water aquifers and coastal ecosystems with nutrient and pathogen pollution from poorly designed systems — even at the beginning of a system’s lifespan.

Onsite wastewater treatment systems also represent a source of greenhouse gases as microorganisms transform wastewater constituents. To understand how microbial communities in the surface soils above shallow drainfields contribute to CH4 and N2O consumption, I measured greenhouse gas surface flux and below-ground concentrations and compared them to the microbial communities present using functional genes pmoA and nosZ. These genes encode portions of particulate methane monooxygenase and nitrous oxide reductase, respectively, serving as a potential sink for the respective gases. I assessed the surface soils above three drainfields served by a single household: an experimental layered passive N-reducing drainfield, a control conventional drainfield, and a reserve drainfield not in use but otherwise identical to the control. Neither GHG flux, below-ground concentration or soil properties varied among drainfield types, nor did methane oxidizing and nitrous oxide reducing communities vary by drainfield type. I found differences in pmoA and nosZ communities based on depth from the soil surface, and differences in nosZ communities based on whether the sample came from the rhizosphere or surrounding bulk soil. Type I methanotrophs (Gammaproteobacteria) were more abundant in the upper and middle portions of the soil above the drainfield. In general, we found no relationship in community composition for either gene based on GHG flux, below-ground gas concentration, or soil properties (bulk density, organic matter, above-ground biomass). This is the first study to assess these communities in the surface soils above a full-scale experimental drainfield.

Overall, my research indicates that coastal communities relying on OWTS must find better ways of understanding short- and long-term groundwater table dynamics to ensure that onsite wastewater systems remain an effective part of the critical water infrastructure in sensitive coastal areas. A substantial proportion of systems in this community are already at risk of poor performance based on compromised separation distance, and the situation is likely to become worse unless stakeholders begin taking a more proactive approach to ensuring system functionality over its lifespan. Designing and installing systems to be more resilient and robust in the face of climate change, while accounting for the accompanying changes in groundwater and precipitation dynamics is critical, as is developing innovative cost-effective solutions to improving wastewater treatment performance. The layered N-removing drainfield I studied is promising in that regard, though my research on microbially mediated GHG emissions highlights the need to consider OWTS’ environmental impacts holistically. Ultimately, we must find affordable and effective wastewater treatment solutions that reduce anthropogenic pollution to both the hydrosphere and atmosphere, or our communities will no longer have access to drinking water resources, nor will its members be able to enjoy the ecosystems they currently rely on for economic, recreational and aesthetic purposes.

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