3.3. Spatial distribution of NO3-N concentrations in shallow
groundwater
The distribution maps for the NO3-N concentration in shallow
groundwater were generated by the Kriging method with selected
semivariogram model (Fig. 3). The map in 2002 showed that the
NO3-N concentrations in the central region, including Tianzhuang,
Xingjia, and Xincheng towns, were high, with a maximal value of
79.74 mg L1. In addition, the NO3-N concentrations in grade VI
were only located in central Tianzhuang town, which had a total
area of 1.76 km2 and accounts for 0.34% of the entire county.NO3-N
concentrations in grade V were mainly situated in central
Tianzhuang town, eastern Xingjia town, and northern Suozhen
town; this total area is 12.88 km2 or 2.48% of the county. The NO3-
N concentrations in grade III and above mostly lay in Tianzhuang
town, southeastern Xingjia town, northern Suozhen town, northeastern
Xingcheng town, eastern Chenzhuang town, and southern
Jingjia town. This area totalled 117.70 km2 and accounted for 23%
of the county. In 2007, high NO3-N concentrations were still found
within the central region, but the range where the maximum NO3-
N value (73.20 mg L1) was increased (Fig. 3). The NO3-N
concentrations in grade VI made up an area of 21.79 km2, which
was 3.85% larger than that in 2002 (Table 3). Similarly, grade V
NO3-N concentrations increased by 12.23% and covered an area of
76.47 km2 in 2007. NO3-N concentrations above grade III (the
drinking water standard 10 mg L1) covered 343.93 km2 and
accounted for 66% of the entire county in 2007, which was nearly
three times as large as that in 2002.