A pond’s watershed is the area of land that contributes water to a pond. The slope of the surrounding upland determines the area that contributes surface runoff to a pond. The shape of the water table around a pond determines the direction that water within the aquifer flows and the ultimate area that will contribute groundwater to a pond.
In the western, hilly areas of the Vineyard, the soil is developed on impervious clay and till. Seepage through this kind of soil is very slow and aquifers may be confined to pockets or beds of sandy material. In these situations, the watershed of a pond like Menemsha Pond, James Pond or Daggett's Pond is defined by the topography that directs the flow of surface runoff to the pond.
The south shore and the north shores east of Tashmoo have very sandy soil and the watershed is not determined so much by the topography of the surrounding upland but rather by the groundwater flow paths. The Commission has prepared a Watershed Map illustrating the approximate groundwater contribution areas to the coastal ponds.
These watersheds define areas where pollutants entering the ground may eventually affect coastal pond water quality. Nitrogen from wastewater disposal, fertilization of landscapes and farm fields and storm water runoff is the primary nutrient of concern. The watershed map is the starting point for determining the impact on water quality of buildout within each pond's watershed.
Stormwater is the water generated from impervious surfaces when it rains. Unless it is dealt with in some other way, stormwater generated near surface waters drives silt, organic matter, bacteria, nutrients, metals, and petroleum products into fresh and coastal waters. The bacteria carried to our coastal ponds contribute to shellfish bed closures that impact our way of life. Stormwater is a visible, often easily corrected source of water pollution.
This source of water quality degradation can be eliminated or at least reduced by having rain and runoff infiltrate directly into the ground, instead of letting it flow into coastal ponds and other surfacewaters. In the ground, it is filtered, treated, and then replenishes groundwater resources.
The use of nonstructural, natural approaches is preferred. Low Impact Development (LID) is an innovative stormwater management approach that avoids costly conventional techniques to pipe, treat, and dispose of stormwater. Instead, it replicates the pre-development hydrology of the site by using design techniques modeled after nature, to infiltrate, filter, store, evaporate, and detain runoff close to its source. Techniques include porous pavers, pervious asphalt, bioretention swales, grassed infiltration areas and rain gardens. LID guidelines should be incorporated into permitting and project approvals at town and MVC levels to minimize the generation of stormwater.
The following criteria are recommended to prioritize which stormwater systems to remediate.
Discharges to surface waters that contain shellfish resources, especially discharges close to the shellfish waters (remove bacteria using vegetative treatment where possible and infiltration to the ground otherwise).
Discharges where nitrogen impairment exists (direct as much stormwater as possible to natural vegetated buffers or artificial vegetated bioretention swales to reduce nutrients, bacteria and other pollutants; infiltrate as much of the remainder as possible).
Where no treatment is possible, the schedule of catch basin clean-outs should be evaluated to determine if a more frequent clean-out is required.
Wastewater is the largest locally controlled source of nitrogen pollution to our groundwater and surface waters. Coastal salt ponds are more sensitive to nitrogen than people, so we need additional measures to limit nitrogen pollution beyond the requirements in place to ensure human health. Wastewater leaving the septic tank contains about 35 parts per million of nitrogen. Natural nitrogen uptake and bacterial conversion to nitrogen gas lowers the septic system nitrogen by about 25% by the time it reaches a coastal pond.
The MVC hosted an Innovative/Alternative Conference on May 12,2016. The goal of this conference was to bring together industry professionals, scientists, policy makers and local stakeholders to discuss the issue of nutrient pollution in our local ponds and estuaries, and to explore various technologies and solutions to this problem. With the wide range of potential solutions discussed, we are confident that our community can prevent further degradation of water quality and take measures to improve the health of our ponds. The MVC will continue to host informational sessions on alternative solutions and we look forward to collaborating to help clean up our waters.Below are links to the presentations given at the conference and a link to MVTV to view the proceedings.
Morning Session link to MVTV site https://player.vimeo.com/video/166645486
Afternoon Session link to MVTV site https://player.vimeo.com/video/167652677
Polly Hill Arboretum Plant Selection Guide:
A resource intended to guide municipalities, landscape architects, and individuals towards plants with excellent horticultural, ecological and adaptive characteristics that are suitable for landscape uses on Martha’s Vineyard.
Alternative Nitrogen Removal Options:
As an alternative to traditional Title 5 septic systems and centralized wastewater treatment systems, there are a number of techniques that can be used to reduce nitrogen at the source or to mitigate its impacts on coastal ponds. This page (under construction) includes links to some of them.
Alternative Septic System Test Center
: The test center, located in Sandwich, MA, evaluates the performance and operations costs of innovative alternative wastewater disposal technologies to provide economical alternatives to conventional septic systems.
National Small Flows Clearinghouse
: Provides objective information and data to communities and individuals to help solve wastewater problems with onsite wastewater collection and treatment systems.