This
method determines the values of individual cells based on a linearly
weighted function, in which known points are inversely weighted
based on their distance from the interpolated cell (closer points receive
a higher weight) raised to a power of 2 (Philip and Watson, 1982). IDW
interpolation was performed using the ArcGIS extension, Spatial
Analyst. The small size of the study areas and the limited number of
data points made it necessary to use all available data points for each interpolation
and to use a barrier to limit interpolation to the immediate
area encompassing the sample locations. The resulting raster surfaces
were classified into equal intervals that were determined by the range
of data values and by the WHO drinking water guidelines or healthbased
limits (World Health Organization, 2011a). Red colors were chosen
to represent interpolated areas that exceed either the WHO guidelines
or health-based limits, while blue colors represent interpolated
areas that fall below these values.