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Use Of Groundwater Model To Study The Interaction Of Coastal Aquaculture And Irrigation Practices On Groundwater Vulnerability

Congress: 2015
Author(s): Saravanan Ramasamy, Balamurugan Ramasubb, Karthikeyan M Sampth
Anna University1, Centre for Water Resources, Anna University, Chennai, India2

Keyword(s): Sub-theme 2: Surface water and groundwater,

Coastal aquaculture is basically farming in brackish water which has contributed substantially to the food security, employment generation and the upliftment of rural poor in coastal areas. It is very essential to ascertain whether the coastal aquaculture has any impact on groundwater. Therefore, a study has been carried out to develop a groundwater model using Visual MODFLOW-MT3D to bring out the real scenario of the impacts of coastal aquaculture on groundwater quality. The scenario's are impact of shrimp farming with out estuary (Scenario-I) and shrimp farming with estuary (Scenario-II). The study area selected is the unconfined aquifer of micro watersheds of Vellar sub-watershed in which the shrimp farming is in practice. The necessary primary and secondary input data were fed to the model (from January 2000 - March 2009) and calibrated for the head (using 2010 observed data). The study revealed that groundwater drains initially towards the Vellar river and then the river flows into the sea and the shrimp farming practice does not have any negative impacts on groundwater quality rather it describes quality deterioration due to over mining of fresh groundwater which led to seawater intrusion. The study also depicted the various locations which were affected by seawater intrusion. This could be overcome by sustainable farming practices like constructing trench around the field and the government can take steps to stop excessive pumping in the fresh water regimes near Bhuvanagiri.

1.0 Introduction

India by virtue of its 8118 km long coastline has 1.2 million ha of potential brackish water area available which is suitable for farming. In addition to this, around 8.5 million ha of salt affected areas are also available, of which about 2.6 million ha could be exclusively utilized for aquaculture due to the unsuitability of these resources for other agriculture based activities (Arthur et al, 2001; Eric Bernard, 2007). But the benefits have been overshadowed by the environmental concerns. In this context it is necessary and essential to picture out the real scenario of groundwater quality in coastal shrimp farming area (Christos et al, 2003). This study will enable to understand the impact of various activities in coastal area including shrimp farming on groundwater quality of the lower Vellar sub watershed located in Cuddalore district which lies between North Latitude 11o25'31'' and East Longitude 79o42'30'' . The area is partially cultivated with paddy and areas near the Vellar estuary are used for aquaculture (Shrimp farms). The Vellar River and its tributaries confluences with Bay of Bengal at various bar Mouth. At these places, the fresh and saline water gets mixed and this water actually makes the land saline and thus land become fallow. Only these fallow lands are being used for shrimp farming in a beneficial way which forms an alternate livelihood option for the farmers (Primavera, 1997). The surface and groundwater sample are collected from different locations for 12 months and these samples were analyzed for physical parameters such as pH, Electrical Conductivity, Total Dissolved Solids, Salinity and chemical parameter using the field kit for the physical parameters and by titration analysis with Silver Nitrate and Potassium Chromate.

2.0 Groundwater Modelling

Visual MODFLOW-MT3D is a standard tool for 3D groundwater flow which combines a water balance equation with Darcy's law and advection-dispersion equation to estimate the plume transport modelling (Kumar, 2002; Saravanan et al, 2006 & 2009). Visual MODFLOW-MT3D is used to simulate the flow and contaminant of the study area for two different scenarios. Impact of shrimp farming with out estuary (refers Scenario-I) and shrimp farming with estuary (refers Scenario-II). Based on the lithological characterization, the aquifer is considered as unconfined with three different layers. The necessary primary and secondary input data (from January 2000 - March 2009) such as hydro geological conditions, physical and chemical aquifer properties including stress were fed to the model. Initially the performance of the model was tested for steady state condition to calibrate the head parameters (using 2010 observed data) and subsequently the model was simulated for transient conditions. The model output are used to reveal the effect of shrimp farming and estuary on ground quality of an unconfined aquifer of the study area based on groundwater flow direction and Chloride concentration.

3.0 Results and Discussion

The results of MODFLOW-MT3D for groundwater flow direction, effect of shrimp faming and impacts of estuary on the groundwater for three different layers shows that the groundwater drains initially towards the Vellar river and then the river flows into the sea. The ground water level is higher in the upstream side of the Vellar river which includes Kovilampondi, Bhuvanagiri and Theethampalayam. The lower level was observed at Port Nova, Vellar Estuary and Nanjampathuvalkai. This depicts the fresh water table is higher in the upstream regions of the Vellar river. Initially the contaminant transport model was performed for Scenario-I considering the chlorine concentration values as species parameter. There is no seepage to layers 2 and 3 because of clay and fine sand formation. In layer 1, the contours of chloride concentration are observed to be in the range of 2,000 to 10,000 mg/l. The high salinity in this region could be due to shrimp farming in the aquaculture ponds and in the Vellar estuary.

Latter the contaminant transport model was performed for Scenario-II. The effect of estuary and shrimp farming on ground water of velar sub watershed in three different layers shows that the concentration is very low in places like Theethampalayam and Killai. The subsequent layers, the chlorine concentration also seems to be very low in these regions (shown in green). This indicates that the areas west to the river are less affected by sea water intrusion. It is found that the intrusion is high in layer 1 (shown in red) compared to the other subsequent layers (2&3). It is observed that, the chlorine concentration around the Vellar estuary have very high values ranging from 5,000 to 25,000 mg/l in layer 1, which is due to point source pollution from shrimp farming and impacts of estuary. Layers 2 and 3 have chloride concentration varying from 2,000 to about 14,000 mg/l just below the shrimp farm area which is not observed in Scenario I. Even though these layers are traced with chloride concentration, it is not purely based on the effects of shrimp farming alone but rather could be due to impacts of estuary and over with drawl of ground water for agriculture. Sustainable farming practices like constructing trench around the field and steps to stop excessive pumping in the fresh water regimes near Bhuvanagiri adopted to address these issues Reference

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2011 IWRA - International Water Resources Association - - Admin