The impact of urban groundwater upon surface water - eTheses ...

WATER QUALITY INTERACTIONS
The Environment Agency has undertaken a survey for 56 pesticides, herbicides and selected
trace organic compounds in water supply rivers, for the Midland Region (Environment
Agency, 1998). The pesticides mecoprop and isoproturon and the organic compounds nonyl
phenol, chloroform and trichloroethene were identified in the sewage treatment discharges
from Ray Hall and Minworth in the upper Tame. In addition to these compounds MCPA was
identified in the surface water at Lea Marston, a distance of 10 km downstream of the study
area. No surface water sampling was conducted within the upper Tame in this survey. No
work has been done on the groundwater contribution of pesticides to the upper Tame, but they
may be present in the groundwater owing to the extensive use of amenity and industrial
pesticides in major conurbations.
7.8 Biodegradation and transport of chlorinated solvents across the
groundwater/surface water interface
The degradation of PCE, TCE and 1,1,1-TCA is indicated by the presence of cis 1,2-DCE and
1,1-DCA within the groundwater. The rate of abiotic transformation is generally low and
most reactions are probably a result of microbially mediated reductive hydrogenolysis (Fetter,
1999). The reduction follows a pathway with decreasing chlorination from PCE to TCE to
DCE to vinyl chloride to ethene (Figure 7.23), and from TCA to DCA to chloroethane to
ethane. Cis 1,2-DCE is only produced as a product of biodegradation and the ratio of
TCE/DCE provides an indication of the extent to which biodegradation has occurred. The
average TCE/DCE ratio varied from 62 in the abstraction wells to 2 in the shallow
groundwater and 4 in the riverbed piezometers. In the case where either only TCE or only
DCE was detected the ratio was calculated using the detection limit for the non-detected
compound. Average values were taken for the north and south bank of each profile to remove
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