September 2012, Vol. 24, No.9

Small Communities

Opportunities for improvement and concerns for conflicts

On-lot and neighborhood stormwater and wastewater treatment considerations

Mary Clark, Amy Macrellis, and Juli Beth Hinds

"Here we go again,” thinks the public works department’s administrative assistant. Spring is here, and the neighborhood complaints have started. Every time it rains heavily, or when the spring snowmelt begins, the phone starts ringing about flooded basements and backed-up septic systems.

One neighborhood with more than 60 residences, all built in the 1950s, has conventional pipe-and-stone leachfields and full basements — the soils are sandy, but have a shallow seasonal water table. During wet periods, the water table rises above some of the leachfields, and basement sump pumps can’t keep up.

“What to do?” considers the public works director. The subdivision is too far from the sewer mains for centralized wastewater service to be cost-effective. Stormwater runoff is not the problem, so traditional storm drainage infrastructure isn’t a viable solution either.

 

Two birds, one stone 

The Town of Colchester, Vt., found a creative solution for both problems: Construct a perforated drain system to lower the seasonal groundwater table, thus eliminating the flooded basements and enabling the wastewater leachfields to function effectively.

The engineering solution is a good one, but it requires careful scientific consideration — wetlands located next to this neighborhood also have to be protected. In a science-meets-engineering resolution, a series of groundwater monitoring wells was installed strategically around the neighborhood and in the wetlands. A sophisticated computer groundwater modeling program studied the problem using real data collected from the monitoring wells. These data enabled a hydrogeologist to make recommendations to the engineers about the location and depth of the drainage system.

The wells were monitored for several years after the drains were installed. The groundwater table was lowered at the home sites while the wetlands remained undisturbed. “Plus,” said the public works director, “the phone has stopped ringing.”

 

Simple design changes with big benefits 

This case study exemplifies the benefits to considering the larger hydrologic and land use picture when making decisions on an individual property — or for an individual neighborhood or development. Not far from the neighborhood described above, a developer designing a progressive neighborhood incorporated stormwater low-impact-development best management practices (BMPs), along with conventional onsite wastewater treatment systems.

The BMPs included infiltration tanks at roof drains and infiltration trenches along roadways, allowing for quick return of stormwater to groundwater. The appropriate state regulatory agency reviewed and approved the stormwater designs, and a separate state agency reviewed and permitted the onsite wastewater designs. The town engineer, looking at all of the systems together, caught the fact that while the leachfield designs and stormwater infiltration features each satisfied the required 0.9 m (3 ft) to the seasonal high groundwater table, there had been no consideration of the combined effects of both systems operating at the same time during wet weather.

Fortunately, the groundwater flow model developed to solve the nearby basement flooding issues could be modified to account for the proposed development’s additional runoff inputs. The model determined that the increase in the groundwater table after a rain could be large enough to affect the operations of the proposed BMPs and onsite wastewater systems.

Simple design solutions, such as raising the bottom elevation of the proposed leachfield trenches and increasing the horizontal separation distances between wastewater leachfields and stormwater infiltration devices, were recommended and incorporated into the final design and construction. Following this instance, the town developed design guidelines so that future development plans would not suffer the same close calls.

 

Translating from site design to policy development 

These case studies illustrate how lessons from the stormwater world easily translate to onsite wastewater management. These lessons often are overlooked when it comes to design, construction, and, most importantly, long-term planning, operations, and maintenance.

While practitioners, regulators, regulatory programs, and engineering disciplines continue to treat stormwater and wastewater as separate disciplines, the practical reality is that there are few differences — and many important commonalities — among the treatment and management approaches used.

The treatment objectives and environmental risks associated with distributed stormwater and wastewater infrastructure components are similar. The core legal and financial issues — how to enable the public sector to manage and fund infrastructure located mostly on private property — are nearly identical, as are many of the program management, financing, and operation and maintenance needs involved in managing distributed water infrastructure.

While the particular constituents in wastewater and stormwater differ, these distributed systems generally involve a mechanism to retain water volumes, settle out solids, provide water quality treatment, and convey treated water at a regulated rate back into a receiving environment. Distributed systems also are far more likely to involve plant- and soil-based treatment, and to have the treatment mechanism located close to the generating source.

In addition, communities are using the management of distributed stormwater and wastewater as a strategy for complementing or replacing conventional centralized systems with smaller-scale, land-based controls.

Beyond managing the hydrologic effects of land development, distributed and naturalized stormwater controls are being used in cities from Philadelphia to Lansing, Mich., to implement watershed restoration and total maximum daily load plans, provide strategic growth capacity while meeting regulatory requirements, and mitigate combined sewer flows.

Similarly, distributed wastewater management programs are in use in settings from rural growth areas and shoreline resort communities to such urban cores as New York City and Mobile, Ala. Decentralized wastewater systems and management programs are used variously to provide wastewater treatment capacity for economic development, strategically protect or augment water resources, reduce demands on combined sewer systems, and expand utility services without extending collection systems or enlarging wastewater treatment plants.

 

Taking a broader look 

A further challenge to practitioners in the onsite wastewater field (and to their counterparts in the stormwater arena) is to apply the lessons and framework of the National Pollutant Discharge Elimination System Phase II program — which has included work on stormwater system inventory and mapping, development of management programs and utilities, and public education and outreach — to the management and funding of onsite systems. A few steps practitioners in both areas can take to learn and apply more include the following:

  • Look to the stormwater field’s municipal separate storm sewer system (MS4) program for a framework to develop management structures that work for all on-lot components.
  • Make sure municipal and regional water policies consider and include both stormwater and wastewater challenges and solutions.
  •  Learn from other programs and professions to have maximum effects and find new and creative solutions that may be affordable and effective.
  • Take a broader land-use and subwatershed view to see if other management options may be more appropriate — sometimes, the neighborhood-scale fix is better and more cost-effective than lot-by-lot upgrades by individual owners.

 

Mary Clark is an environmental analyst in the Drinking Water and Groundwater Protection Division of the State of Vermont. Amy Macrellis is a water quality specialist at Stone Environmental Inc. (Montpelier, Vt.). Juli Beth Hinds is principal of Birchline Planning LLC (Rutland, Vt.).

 

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