Page 256 - Hydrogeology Principles and Practice

HYDC06 12/5/05 5:34 PM Page 239

Groundwater quality and contaminant hydrogeology 239

Fig. 6.25 The nitrogen cycle showing
sources of nitrogen incorporated in the soil
zone and the principal pathways of nitrate
production (mineralization) and
consumption (denitrification). Any excess
nitrate not taken by plants and microflora
or removed by denitrification can
potentially leach to groundwater below
the soil zone.

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6.4.3 Agricultural contaminants ards (50 mg L as nitrate) are concerned with meth-
aemoglobinaemia (‘blue baby’ syndrome) in infants
Agricultural contaminants include nitrate and pesti- (Walton 1951; Craun et al. 1981) and gastric cancer
cides used in intensive farming practices that often (National Academy of Sciences 1981; Nomura 1996).
affect wide areas of aquifer outcrop. As such, nitrate Environmental impacts associated with excessive
and pesticides are diffuse pollutants in the environ- nitrate in the aquatic environment are eutrophication
ment and can lead to serious consequences for the of inland and coastal waters and the consequent loss
quality of groundwater resources and surface waters of biodiversity (Hecky & Kilham 1988; European
receiving contaminated groundwater. Other sources Environment Agency 2003). Approaches to control-
of contamination arise from livestock and poultry ling diffuse contamination of groundwater by nitrate
farming through the intensive management of graz- include ‘end of pipe’ technological solutions such as
ing pasture and the operation of concentrated animal blending a contaminated source with a low-nitrate
feeding operations (COFAs) (Mallin & Cahoon 2003). water, biological reactor beds and anion exchange
Nitrate contamination of groundwater has resulted resins (Hiscock et al. 1991). To prevent further con-
from the desire for greater self-sufficiency in food tamination and achieve environmental standards,
supply that has led to the ploughing up of grassland as for example set by the EU Nitrates Directive, ap-
and the application of nitrogen-based fertilizers and proaches include reductions in fertilizer and manure
organic manure. Ploughing up grassland stimulates applications following good agricultural practice and,
the natural process of mineralization and the release more radically, changing land use from arable cropping
of nitrate from the organic-bound nitrogen in the to low intensity grassland or forestry (Section 7.3.4).
soil zone (Fig. 6.25). The use of fertilizers during the Pesticides refer to the group of synthetic organic
period of crop growth often coincides with the onset chemicals used mainly as fungicides, herbicides
of the rainfall season and the use of irrigation water. If and insecticides. Herbicides are used in the largest
fertilizer and manure applications are not applied fol- quantities and generally have much greater water
lowing good agricultural practice (see Section 7.3.4), solubility compared with insecticides such that criti-
soil nitrate leaching losses can occur causing high cal concentrations, in excess of water quality guide-
nitrate concentrations in the unsaturated (Fig. 4.10b) lines and standards, may be exceeded. The European
and saturated zones (Box 6.6) of aquifers. Union has adopted a maximum admissible con-
The health effects of high nitrate concentrations centration of 0.1 µgL −1 for any individual pesticide
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in excess of European and WHO water quality stand- and 0.5 µgL for the sum of all individual pesticides,