Pollution Prevention - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 454

bacteria; parking lot runoff has oil, metals, and antifreeze; construction sites release soil;
and industrial facilities may have on hand barrels of hazardous material capable of being
swept away in a flood. It is important to know the locations of the hazardous substances
and their relationship to the ephemeral drainage network capable of transporting their
releases. For example, some erosion may reach a catch basin 30 m (100 ft.) away, yet in other
cases the eroded material will never make it to a storm drain because it reaches a natural
depression first. Aerial photos and topographic maps can help identify potential routes of
overland flow; field checking, however, is required for verification. In addition, dye testing
is often used to help identify infiltration/inflow of precipitation into sewers and to reveal
the paths of stormwater and wastewater. During this process, nontoxic dyed water is intro-
duced into roof drain leaders, driveway drains, or area drains. In some instances, dyed
water is injected into the ground around foundations to check for the illegal connection
of foundation drains. After introducing the dyed water, the downstream sanitary sewer
access structure is checked for its presence to ascertain the path.
13.4.1.3 Step 3: Assessing CRFs of Substances Present
The locations where the highest CRFs are present should now be evaluated to help with
the prioritization of the pollution prevention efforts. The results of the water sampling
should guide the selection of locations. For example, if aquatic ecosystems have already
been damaged and the groundwater samples produce high levels of chromium, this con-
taminant needs to be addressed as a priority.
13.4.1.4 Step 4: Preliminary Assessment of Geologic Vulnerability
The vulnerability of soils to the discharge of stormwater also needs to be considered. Some
suggestions for assessing the soil vulnerability in an urbanized watershed include
• Do not rely upon the county soil surveys—use them for general orientation to
the soils in the watershed. Urban soils are highly disturbed, and frequently the
soil surveys do not capture their variation because the survey may be 10-20 years
old.
• Instead, conduct a soil sampling program that enables a correlation between soil
type, infiltration rates, and routes of overland flow during wet-weather events. The
objective is to reveal those locations in the watershed where larger-grained vul-
nerable soils and less-vulnerable fine-grained soils exist. Use the soil data avail-
able from environmental site investigations to help determine the locations for
additional sampling (Chapter 4). Once the sites for sampling have been selected,
compare their locations to the major pathways of overland flow in the watershed,
such as steep unvegetated slopes, large parking lots, and major streets. Then
sample those sites within or along the drainage paths. At the sampled sites, use
an infiltrometer to measure the infiltration capacity of the soil and have the soil
samples analyzed for type, texture, and organic content by the local agricultural
extensions of the nearest state university.
• Delineate floodplains and other areas where groundwater levels are likely to be
closer to the surface. Try to avoid the placement of infiltration activities in these
zones and concentrate on areas where the depth to groundwater is higher, thus giv-
ing more time for organisms and structures in the soil to reduce the contamination.