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Groundwater quality and contaminant hydrogeology 241

and in North America guidelines are applied to indi- Factors explaining the distribution of herbicides in-
vidual pesticides (Appendix 9). The factors affecting cluded an inverse relation to well depth and a positive
the leaching of pesticides from soils include the correlation with dissolved oxygen concentration that
timing of application, the quantity reaching the tar- appear to relate to groundwater age, with younger
get area and the physical and chemical properties groundwater likely to contain herbicide compounds.
of the soil. Newer formulations of pesticides are The occurrence of herbicide compounds was sub-
tailored to have short half-lives of less than 1 month stantially different among the major aquifer types
in the soil through retention and elimination of com- across Iowa, being detected in 83% of the alluvial,
pounds by hydrophobic sorption (see ‘Hydrophobic 82% of the bedrock/karst region, 40% of the glacial
sorption of non-polar organic compounds’ in Section till and 25% of the bedrock/non-karst region aquifers.
6.3.3) and degradation by chemical hydrolysis and Again, the observed distribution was partially attrib-
bacterial oxidation. Caution is required, however, uted to variations in groundwater age among these
in that quoted half-lives appropriate to a fertile clay- aquifer types. A significant, inverse relationship was
loam soil may not be representative of permeable identified between total herbicide compound con-
sandy soils developed on aquifer outcrops. Below the centrations in groundwater and the average soil slope
soil zone, pesticide mobility will again be affected within a 2-km radius of the sampled wells. Steeper
by the availability of sorption sites for attenuation soil slopes may increase the likelihood of surface
and the viability of micro-organisms for bacterial runoff occurring rather than transport to groundwa-
degradation. ter by infiltration (Kolpin et al. 1997).
Sorption is promoted by organic carbon, iron oxides In the United Kingdom, isoproturon is the most
6
and clay minerals and is a significant mechanism in extensively used pesticide with over 3 × 10 ha trea-
the attenuation of pesticides with depth such that ted in 1996 (Thomas et al. 1997). Concentrations of
the amount of pesticide leached to groundwater is isoproturon greater than the European Union limit
generally less than the amount lost to surface runoff have been found in groundwater abstracted from
(Rodvang & Simpkins 2001). Total herbicide losses in the major Chalk aquifer (Table 6.10). Although con-
subsurface drainage on fine-textured soils are usually centrations are generally low there is concern that
less than 0.3%, but occasionally 1.5% of the amount significant quantities of isoproturon may be moving
applied. through the unsaturated zone only to contaminate
Contamination of groundwater by pesticides is groundwater in the future. Clark and Gomme (1992)
common in agricultural and urban areas. In a survey recovered unsaturated Chalk cores for pore water
of groundwater in 20 of the major hydrological basins analysis and showed that the uron herbicides (isopro-
in the United States in which 90 pesticide compounds turon, chlortoluron and linuron) left the base of
(pesticides and degradates) were analysed, one or the profile at very low concentrations (Table 6.11)
more pesticide compounds were detected at 48% of and had not penetrated beyond 2 m into the unsatur-
the 2485 sites sampled. The pesticide concentrations ated zone. If correct, these results would suggest that
encountered were generally low, with the median pollution of Chalk groundwater pollution by the uron
−1
total concentration being 0.05 µgL . Pesticides were herbicides through intergranular flow in the Chalk
commonly detected in shallow groundwater beneath matrix is unlikely except in areas where the water
both agricultural (60%) and urban (49%) areas and so table is close to the surface. Where uron herbicides
highlighting urban areas as a potential source of pesti- are detected in Chalk groundwater, it is possible that
cides (Kolpin et al. 2000). pesticide transport has occurred by flow through the
In Iowa, which has some of the most intensive ap- fissure system. Support for these results is provided
plications of herbicides in the United States, herbicide by Besien et al. (2000), who measured recovery rates
compounds were detected in 70% of 106 municipal of isoproturon of 48–61% in laboratory column ex-
wells sampled; with degradation products compris- periments using Chalk cores eluted with non-sterile
ing three of the four most frequently detected com- groundwater containing an initial mass of 1.5 mg of
pounds (Kolpin et al. 1997). The highest herbicide isoproturon. The column results also illustrated the
concentrations in groundwater were found in areas importance of microbial degradation in removing
of greatest intensity of herbicide use (Table 6.9). isoproturon during the 162-day experiment.