Water Resources 10.3: Stormwater

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 surface waters. In the ground, it is filtered, treated, and then replenishes groundwater resources.

The use of non-structural, natural approaches are 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, 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.

1)   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),

2)   Discharges where nitrogen impairment exists (direct as much stormwater as possible to natural vegetated buffers or artificial vegetated bio-retention 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.

Objective W4: Eliminate or reduce direct discharge of stormwater runoff into sensitive water resources.

It should be possible to achieve a one third reduction of direct stormwater discharges from the downtown collection systems in Tisbury, Oak Bluffs, Edgartown and Menemsha.

·  Strategy W4-1:  Set up a program to identify and correct problematic stormwater discharges from roads and other public lands. The first step is for the towns and MVC to map the most problematic existing discharges into wellheads, wetlands, streams, ponds, harbors, and the ocean. Use the 2003 (CZM funded) Stormwater System Mapping Project report for Oak Bluffs and Tisbury as a model to devise a similar plan for the other Towns.  The volume of direct runoff to surface waters from current stormwater collection discharges in Tisbury, Oak Bluffs, Edgartown and Menemsha can be reduced by infiltrating up-gradient segments to the ground. Discharges from Town and State roadways where they cross streams should be identified, evaluated and corrected.  The road surface at each crossing should be adjusted during repaving to divert as much runoff as possible into roadside vegetation or leaching catch basins before it reaches the stream crossing. Road salting activities near the stream road crossings and within the catchments of the stormwater collection systems that discharge directly to streams should be limited to using deicing products with the least impact.

·  Strategy W4-2:  Require development and redevelopment projects to maximize treatment and infiltration in order to retain all stormwater on site, favoring use of Low Impact Development techniques. Stormwater management on private property can be improved through education, incentives, and regulations aimed at maximizing retention and infiltration of stormwater on the property where it is generated in a manner that maximizes the removal of bacteria, metals and nutrients.  New stormwater discharges that will add runoff to existing collection systems that now discharge to surface waters should be prohibited.  Stormwater systems should capture and treat at least the first ½ inch of runoff. A 24-hour, 25-year return storm is suggested as the goal and a10-year return storm as the minimum acceptable for on-site disposal design.  Stormwater management will also benefit groundwater supply by maximizing recharge. Require private-project stormwater system management plans and adherence to maintenance schedules to assure optimum system performance.  Within the Zones of Contribution of public water supply wells, impervious surface areas should be limited to less than 2500 square feet or 15% of the lot; similar limits should be implemented within private drinking water well areas.

Strategy W4-3:  Put in place system design and maintenance programs to limit stormwater problems. Utilize natural vegetation, and if natural infiltration is not possible, use systems that offer some nitrogen removal capability for projects that are in the watersheds of nitrogen-sensitive coastal ponds.  All stormwater systems must have an operation and maintenance plan and a system to ensure that regular maintenance of catch basins and vegetated systems is carried out and to ensure consistent maintenance of runoff management over time.  Any stormwater systems that have the potential to discharge hazardous materials to ground or surface waters must have an emergency shut-off system that will prevent such contamination. In areas where there are drinking water wells or for impervious areas that contribute to surface water discharges, oil-absorbing pads or stormwater treatment units that remove oil should be used.