Evaluating the Impacts of Sea Level Rise on Coastal Rhode Island Septic Systems and the Effectiveness of Vegetation-Based Mitigation Strategies.

Coastal ecosystems along southern Rhode Island face threats from both excess nutrient loading and sea level rise. Together with increasing storm-related tidal surges, sea level rise is causing coastal groundwater tables to rise, though precisely how coastal aquifers and groundwater tables are likely to respond to these events is still poorly understood. Research has shown that raised groundwater tables interfere with the ability of onsite wastewater treatment systems (OWTS) to adequately treat wastewater, resulting in increased nutrient loading in ground and coastal waters. OWTS leachfield design is predicated on a minimum separation distance between the infiltrative surface and the groundwater table, since a certain volume of soil is required to adequately treat septic tank effluent before it reaches groundwater aquifers. If groundwater tables are rising as a result of sea level rise and climate change, coastal OWTS may no longer be functioning optimally, and may be contributing significantly to the degradation of coastal waters through nutrient and pathogen loading. This study has two major aims: (1) quantify variations in groundwater tables and their effect on separation distance under OWTS in coastal Rhode Island, and (2) determine whether perennial herbaceous plants can be used to lower nutrient content and water volume of wastewater before it reaches the groundwater. In light of sea level rise, we expect to see that older systems – constructed to meet the 90cm groundwater separation distance regulations at time of installation – will show a decrease in distance from the leachfield infiltrative surface to the groundwater table. Additionally, we expect to find that deep-rooted fast-growing grasses and perennials can reduce some of the water and nutrient load of the wastewater infiltrating into the soil in leachfields during the growing season. We expect that plants may boost phosphorus and nitrogen removal rates in OWTS, when compared to systems without plants.