groundwater irrigation and its impact on hydrology groundwater irrigation and its impact on...
Post on 20-May-2020
Embed Size (px)
CHAPTER - FIVE
GROUNDWATER IRRIGATION AND ITS IMPACT ON HYDROLOGY AND MORPHOLOGY
Murshidabad District is one of the worst affected areas of arsenic contamination
in groundwater in the world. The other affected areas are Malda, Nadia, and North and
South 24 Parganas districts of West Bengal. According to the water test report of Public
Health Engineering Department in Murshidabad District 28,357 water samples have
arsenic concentration above 0.05 mg/l in groundwater. 22 out of 26 blocks have arsenic
concentration 9(Samaddar and Subbarao, 2007). This district is also one of the worst
affected areas in the world by arsenicosis.
6Abhijit Mukherjee et. al., 2007 had documented the groundwater flow and a
regional- scale hydrostratigraphy for a ~ 21,000 km2 area of the arsenic affected
districts of West Bengal. These districts include Murshidabad District, Nadia, North 24-
Parganas and South 24- Parganas (including Kolkata). Eight 22- layer model scenarios
of regional groundwater flow were developed based on the observed topography,
seasonal conditions and inferred hydrostratigraphy. A comparative picture is drawn
between the existence of seasonally variable, regional north- south flow across the basin
prior to the onset of extensive pumping and severe pumping, therefore, distorting the
flow pattern, inducing high vertical hydraulic gradients across wide cones of depression
after 1970’s, a landmark period of Green Revolution. Downward flows of irrigational
return flow and inflow from rivers have resulted in arsenic contamination in shallow
ground water. Previously safe aquifers appear to have contaminated by a combination
of mechanical mixing and chemical equilibrium.
5.1 FACTS AND FINDINGS ABOUT ARSENIC IN GROUNDWATER IN THE DISTRICT
Stuben et.al. 13(2003) had made a study in a small watershed at and around
Berhampore block in ‘bagri’ region of Murshidabad District. Groundwater from wells
and the main rivers crossing the area (Bhagirathi and Gobra) have been sampled in the
north of Murshidabad District covering an area about 200 square kilometers. The
concentration of major cations including Ca, Mg, K, Na and Fe were measured.
Fluoride, Cl-, NO3 - and SO4
-2 concentration were also determined. According to the
relative molar portion of the dissolved ionic species, all samples are of Ca – HCO3 type.
River and groundwater samples are similar in their main hydro-geochemical
compositions, but the total amount of dissolved solids in river water is slightly lower in
Bhagirathi about 300 mg/l, Gobra about 500 mg/l and ground water is 700 mg/l
symptomatic of anthropogenic interference into the groundwater through soil due to
infiltrating, surface run-off. In this chapter the geo-chemical mechanism of arsenic
contamination in groundwater has been discussed, and emphasis on the effect of
changing land-use on groundwater since Green Revolution.
Mobilization of As (arsenic) in groundwater is mainly governed by low
temperature geochemical processes that involve leaching of naturally occurring ores.
Accumulation of As (arsenic) in soil and groundwater especially under anoxic
conditions, due to anthropogenic input or human activity (Bowell et. al., 1994; Azcue
et. al., 1995; Mariner et. al., 1997; Chakraborty et. al., 1998; Ashley and Lottermoser,
1999; Madhavan and Subramaniam, 2000; Stuben et.al. 2001 in 9Stuben et.al. 2003).
According to Mallick and Rajagopal, 1996; Das et.al, 1996; Mandal et.al. 1996;
Chowdhury et. al.; 1999) arsenic is released to the groundwater from sulfides mostly
pyrite which were oxidized primarily due to water level drawn down as a consequence
of intensive irrigation and groundwater exploitation. A group of scientist opines that
arsenic is released by the oxidation of pyrite or arseno-pyrite following the lowering of
water ground water level. The other view is that arsenic is released due to desorption
from or reductive dissolution of ferric oxy-hydroxide in reducing aquifer in
environment (KMPC, 2006). The widely accepted hypothesis by a group of researchers
from the Division of Land and Water Resources, Royal Institute of Technology,
Sweden is, arsenic is fixed on Fe-oxyhydroxides (FeOOH). Fe-oxyhydroxides
scavenges and immobilized arsenic in an aerated aquatic system but in the presence of
organic material and as a released; again due to the microbial mediated reductive
dissolution of Fe-oxyhydroxides arsenic is released from such Fe- oxyhydroxides under
reducing condition. The flooding method of irrigation gradually sealed the aquifer from
diffusion of oxygen.
A.K. Singh, (2006)11 has explained the organo-arsenic cycles in sediments with
a flow diagram. He had explained how natural arsenic rises in quantity in groundwater
due to anthropogenic factor.
Figure no.: 5.i Organo-arsenic cycles in sediments (after Singh A.K., 2006)
Concern has been raised by Bangladeshi and international scientist about
elevated levels of arsenic in Bengali food particularly in rice grain cultivation. The
highest level of As (arsenic) in the southwest of Bangladesh (district -Nawabganj,
Faridpur, Rajbari and Gopalganj) share a common international boundary with
Murshidabad District of West Bengal therefore the soil and aquifer are quite similar. In
the rice survey both wet season crop ‘aman’ and dry season crop ‘boro’ were sampled
and observed. 8Roy Chowdhury et.al., 2002, surveyed Domkal blocks in Murshidabad
District where the arsenic level is above 0.05 mg/l in many places. According to the
study many million cubic meter of groundwater is extracted daily by shallow big
diameter tubewells for agricultural irrigation. The mean concentration of As is 10.7
mg/kg, Fe is 7860 mg/kg and Mg is 733 mg/kg in higher side of agricultural land soils
compared to the fallow land soils and lower sides of the agricultural land soils. This
arsenic indirectly gets included in the daily food chain of the common people. Those
consume less protein in their daily food habit are worse affected by intestinal problems.
Table no. 4.xvi shows the case study of arsenic contamination in food items in Domkal
and Jalangi block of Murshidbad District in a report by The National Institute of Health
Sciences, Japan, published in journal of Centre for Science and Environment (CSE),
Table no: 5.i Mean concentration of arsenic in food items in Domkal, Jalangi block.
Species As concentration micron/gram
Tumeric 0.27 to 0.43
Source: Roy Chowdhury et.al. 2002, pp: 605-618.
Groundwater occurs under unconfined conditions particularly in the ‘bagri’
region of Murshidabad District. The ‘rarh region shares partly unconfined and partly
semi-confined aquifer condition. The unconfined aquifer occurs along the tectonic
troughs through which the major rivers flow. Arsenic gets contaminated within 20-80
meters intermediate aquifer within the meander belt of the upper deltaic plain. The
aquifer in the West Bengal part of the basin probably belong to a) Late Pleistocene to
Holocene Ganges sediments and b) Early to Middle Pleistocene coastal and moribundh
Ganges delta deep aquifers composed of stacked, main- channel, medium to coarse
sands at depths more than 130 m.
Mukherjee, et. al. (2007), had tried to explain the flow and mechanism of groundwater
due to irrigation since the introduction of green revolution. The extensive exploitation
of groundwater after 1970’s in the up–gradient of mature delta, flushes and dilutes the
arsenic present at very slow rate. The regional flow occurs within the major system
from north to south down- gradient during the dry season probably due to the low
topographic gradient (~0.1 m/km). Very frequent pumping centers in the agricultural
fields and aquifer architectures dictated the hydraulic gradient of the study area. The
absence of continuous confining layers, pumping induces between relatively shallower
and deeper groundwater, which may lead to arsenic enriched water to spread downward.
The arsenic contaminated water when spread over the land surface partially gets
accumulated in crops and partially re-circulate in to the sub surface via enhanced
recharge. This mechanism of repeated extraction and recharging may worsen the
scenario of Murshidabad District in future.
Mukherjee (2006), has opined that the smaller rivers in the study area were
found to be mostly gaining from groundwater discharge. Presumably the groundwater
discharge to rivers has decreased because of pumping by deep tubewells and shallow
tubewells. As a consequence, the outflow from the shallow aquifers may have impacted
the chemistry of smaller rivers. River Bhairab, Dwarka, Pagla, Mayurakhi and many
other rivers may have the same story.