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aquifer depth, spatial lithological changes, and the anthropogenic activities in
recharge areas.
Loftis (1996) reviewed national assessments, agricultural, urban, point
source and hazardous waste case studies on regional and localized groundwater
quality all over the world, including a few snapshots. Based on this review,
the correct meaning of 'trend' was emphasized as a critical step for groundwater
quality studies in both temporal and spatial context. Generally, trends are
thought of as changes over time at either a regional or local spatial scale and
the water quality managers are mostly interested in changes due to artificial
activities. It was concluded that although there are many regional groundwater
studies, which provide a 'snapshot water quality description over an area at
a time, only few consider temporal changes and still fewer include a statistical
analysis of long-term trends.
Lee and Lee (2003) evaluated and quantified the potential of natural
attenuation of groundwater at a petroleum-contaminated site in an industrial
area of Seoul, Korea. Eight rounds of groundwater sampling and subsequent
chemical analyses were performed for a period of three years. Groundwater of
the study area was found contaminated by toluene, ethylbenzene and xylene
(TEX). TEX concentration was found decreasing with time, with the TEX
plume in a quasi-steady state. The trend analysis by the Mann-Kendall Test
along with the changes in mass flux and plume area confirmed that the TEX
plume reached a quasi-steady state. Furthermore, the proportion of the total
attenuation attributable to biodegradation was found decreased during the
monitoring period, while the contribution of other attenuation processes
(dilution or dispersion) was found increased.
Kim et al. (2005) applied time series analysis to investigate the effect of
tide on groundwater quality in a coastal area of Korea. Continuous and regular
in situ monitored data of electrical conductivity (EC) and groundwater level,
and tidal level data measured by the National Oceanographic Research Institute
were used for the time series analysis. It was found that EC and groundwater
level conspicuously fluctuate with two periodicities (15.4- and 0.52-day),
which is very similar to those of the tide. Also, the behaviours of their
fluctuations vary in accordance with the tidal period. It was concluded that the
groundwater quality is mainly controlled by the tidal level, and the strength of
tidal effect on the groundwater quality is different according to the tidal
period.
6.5 Time Series Analysis of Irrigation Requirement
and Soil Moisture
Gupta and Chauhan (1986) studied the stochastic structure of weekly irrigation
requirements of a crop by considering the irrigation requirement time series as
an additive model with trend, periodicity and stochasticity as its components.
Each component was identified and, if found, removed from the original
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