Sunday, April 5, 2009

Lake Whatcom Watershed Infiltration Pilot Project

The Bellingham City Council has proposed a Lake Whatcom Watershed Infiltration Project to develop a pilot comprehensive residential on site stormwater reduction, infiltration, and reuse plan for the City's portion of the Lake Whatcom Watershed.

The pilot is to consist of 15 volunteer homeowners with homes that represent the most promising prospects for a reduction in impervious and semi-pervious surfaces, infiltration of stormwater that would otherwise be directed offsite, and stormwater reuse in households and gardens.

The program is to be in addition to the current rain barrel program and should consider strategies such as removal of pavement, installation of Bioswales, rain gardens, pervious pavement, creation of habitat enhancement pools, roof infiltration drainage systems, strategies for storage and reuse in households and gardens and similar efforts.

The city will cover all costs for implementing the pilot program at no cost to the volunteer homeowner. The maximum allowable expenditure for construction or renovation of a homeowners property shall be $8,500.

The program shall be implemented in 2009, and, if successful, should be expanded in 2010 to include more volunteer homeowners in the watershed.


Individual residential systems can provide economical onsite treatment of low impact residential stormwater runoff on a single property. Terrific idea, if everyone is treating their own stormwater runoff.


Individual residential systems by themselves are not effective stormwater management tools. One downstream house cannot be expected to provide treatment for 100 upstream houses or pretreat runoff to prevent pollution or sediments from entering the lake.

The City and County must treat major stormwater runoff sources, like ditches and housing developments that drain directly in to Lake Whatcom. There are no magic fixes. Homeowners can not fix this problem by themselves.

Infiltration projects can introduce contaminants like nitrate, chloride, gasoline and other heavier, less volatile, but very soluble hydrocarbons into soils and ground water.

An infiltration basin or trench will not reduce dissolved oxygen concentrations in stormwater.

The success of infiltration structures depend upon how much stormwater is diverted to ground water.

The ability of an infiltration program to capture nutrients depends on the soil and the basin’s detention volume. The Silver Beach neighborhood is located at the bottom of steep hills and mountains.

Infiltration structures should include provisions for pretreating the water to prevent premature clogging of the basin. At this time, many newer residential developments on the East side of the lake release stormwater runoff directly to the lake without any pretreatment of any kind.

It is well documented that the combination of pretreatment and infiltration removes the greatest amounts of pollutants.

Infiltration basins are commonly used for drainage areas of 5 to 50 acres with land slopes less than 20%.

Infiltration systems by themselves are not sediment control devices. The size and location of the infiltration basin must be adjusted to provide for removal of sediment particles in a pretreatment unit.

Pretreatment is required for all infiltration basins that receive any stormwater containing particulate matter or pollutants that might clog the infiltration structure or leach to groundwater.

A major part of the City’s strategy to control stormwater runoff is to treat runoff from residential rooftops and lawns.

But runoff from residential rooftops and lawns is considered the least polluted of urban runoff and therefore the safest runoff for discharge into infiltration structures and eventual return to groundwater. In fact, residential runoff requires less pretreatment prior to infiltration, as long as care has been taken with fertilizers, herbicides and pesticides.

Infiltration trenches:

An infiltration trench or ditch is an excavation that is 2 to 10 feet deep, often lined with a sand base or gravel with fines, a protective layer of filter fabric on the sides and filled with course stone aggregate.The empty spaces between the gravel provide temporary storage of runoff, with runoff making its final infiltration through sub-soils at the bottom of the trench.

Depending on their size, infiltration trenches can divert up to 90% of the annual runoff into the soil.

Infiltration trenches should not be located in areas where watershed slopes are 20% or greater. Slopes less than 5% are preferred, which would seem to rule out most of the Lake Whatcom Watershed.

Other interesting facts:

Trenches with sump pit pretreatment last longer than trenches with grass filter strips for pretreatment.

Overflow: A diversion path rather than an emergency spillway should be used to pass excess surface runoff over the trench to a waterway.

In some cases, the EPA has defined infiltration trenches (also includes traditional dry wells and French drains) as Class V injection wells. Injection wells for disposal of pollutants are prohibited in some states.Without pretreatment, infiltration and injection can pollute host properties and ground water of adjacent properties, creating additional public health hazards.

Meanwhile, in East Silver Beach, water is literally flowing out of the ground onto low lying basin properties (including properties that are equipped with older versions of infiltration trenches).

A sign that hopefully signifies that it is time for the city and county to address stormwater runoff and ground saturation issues.

Potential ground water and soil contamination issues exist if pretreatment is not included for infiltration projects.

Metals and many nutrients are captured in the first foot or two of soil, some soluble pollutants are capable of traveling much greater distances. Pretreatment can remove sediment, oil and grease.

Other factors that indicate potential failure are sites with high pesticide - herbicide or pathogen levels.

Potential contaminants include but are not limited to:

Nutrients, especially nitrates

Salts, especially chloride

Volatile organic compounds, or VOCs.

Pathogens, especially enteroviruses, along with other pathogens sucha as Pseudomonas aeruginosa, shigella, and pathogenic protozoa)

Bromide and total organic carbon (ozone or chlorine treatment can help)


Other organics, 1,3 dichlorobenzend, pyrene, fluoranthene, benzos, phthalate, pentachlorophenol, and phenanthrene.

Heavy metals, chromium, lead, nickel and zinc.

(Pitt, R. et al. 1994)

I urge the City of Bellingham to install additional pretreatment stormwater facilities to prevent pollution of residential properties and Lake Whatcom.

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