A Framework For Ground Water Quality Modelling In The Coastal Aquifer Of Netravathi and Gurpur River Confluence

Abstract

Coastal river confluence is vulnerable to the degradation of groundwater quality around the World. Since the primary source of groundwater is precipitation, the chance of contamination of groundwater is very less compared to surface water bodies like rivers, streams and lakes. Even though the soil has filtering capacity, it overloaded by the excess amount of pollutants, which automatically leads to groundwater pollution. The present study investigates the status of groundwater quantity and quality in the coastal river confluence of Netravathi and Gurpur, which lies near Dakshina Kannada district of Karnataka state on the West coast of India. The study area is bounded by Arabian sea in the West and river Gurpur in the North and river Nethravathi in South with an areal extent of 140 km2. The proximity to the sea, growing population and high demand for groundwater and climate changes make the area vulnerable to the decreasing and degradation of groundwater quantity and groundwater quality. In this regard, the groundwater samples are collected, tested and statistically analysed for groundwater quality. The groundwater head and groundwater quality in the study area is modelled using numerical groundwater flow and transport model and the contamination distance from the coast is assessed. In this study, the field investigation was carried out to identify the aquifer characteristics of the study area. The formation is a shallow unconfined aquifer which consists of lateritic soil. The well samples are collected from different locations of the confluence of Netravathi and Gurpur river with well depth varying from 3 m to 20 m below the ground level. The aquifer parameters in the study area are identified from the pumping test. The pumping test results are analysed for aquifer parameters such as transmissivity, specific storage and hydraulic conductivity. The pumping test results show that transmissivity values are ranging from 241.56 m2/day to 950.4 m2/day and specific storage ranging from 0.000107 to 0.000197 respectively. The transmissivity and the hydraulic conductivity calculated from pumping test are used for the groundwater flow and transport modelling. The field investigation is carried out to collect the groundwater samples and groundwater level data from January 2013 to December 2014 and April 2016 to May 2017 on a monthly basis. The groundwater samples are tested in the laboratory to find the status of the groundwater quality over the study region. With the help of ArcMap 10.2, groundwater quality maps are generated to represent the spatial and temporal variation of quality parameters. Theii groundwater level data is used as an input data for groundwater flow model and groundwater quality data used as an input data for groundwater transport model. Statistical analysis of Sodium absorption ratio (SAR), piper plot, Two-tail significant test, factor of sea correlation, groundwater quality status, prediction of significant chemical parameters and geostatistical methods of groundwater quality mapping for the month of April and May 2016 are carried out to know the status of the groundwater quality. The SAR result shows that the groundwater quality has no contamination of SAR parameters and the quality status is well within the permissible limit for the month of April and May 2016. The piper plot also shows good groundwater quality in the month of April and May 2016 even though a slight increase in the concentration of groundwater quality parameters is observed which infers a chance of contamination in the future. In the 2-tailed significant test, the groundwater quality parameters EC, TDS, Cl and Ca are strongly correlated for the month of April and May 2016. For the month of April and May 2016, the groundwater quality maps for the Thumbe and Maripal wells shows excellent groundwater quality. From the maps, it can be observed that Panganimuguru and Kunjatbail wells are of poor groundwater quality. In the statistical analysis, the present scenario of groundwater quality status is within the permissible limit of the drinking water standards. Even though the quality is under the permissible limit, the trend of groundwater quality shows an increase in the concentration of the groundwater quality parameters beyond the permissible limits, imposing a threat of future contamination. Thus, the groundwater flow and transport model are developed and run for groundwater quantity and quality using FEMWATER, which in the three-dimensional Finite elements (FEM) coupled in Groundwater Modelling System (GMS 10.0). The groundwater flow model and groundwater transport model are run for both steady state condition and transient state condition for a time period of September 2013 to May 2017. In steady state condition, the R2 value of the groundwater head is found to be 0.98 for calibration and 0.9 for validation respectively. In the transient state condition, the model is simulated for calibration with a time period of 486 days (September 2013 to December 2014) with a constant time interval of 30 days. In the validation, the model is simulated with a time period of 425 days (April 2016 to May 2017) with a constant time interval of 30 days. In transient state condition, the R2 value of the simulated groundwater head and observed found to be 0.86 for calibration and 0.86 for validation. The groundwater flow model has better performance since the R2 value is found to be above 0.85.iii In the groundwater transport model, the model is run for both steady state condition and transient state condition for different groundwater quality parameters such as Cl, TDS and Bicarbonate. In the steady state condition, the R2 value obtained for the groundwater quality parameters namely Cl, TDS and Bicarbonate are 0.94, 0.9 and 0.88 respectively. In the transient state condition, the model is calibrated for a time period of 486 days with a constant time interval of 30 days. The R2 value of the transient state calibration of the groundwater quality parameters Cl, TDS and Bicarbonate are 0.92, 0.85 and 0.87 respectively. The model validation for transient state condition is validated for a time period of 425 days with a constant time interval of 30 days. The R2 value of the transient state validation of the groundwater quality parameters Cl, TDS and Bicarbonate are 0.88, 0.95 and 0.93 respectively. The results infer that the model performs better for groundwater transport model. The transient validated groundwater transport model is then considered for prediction. The prediction scenarios are classified based on the recharge and injection wells inflow rate. The recharge is further classified into three scenarios. (i.e.,) Minimum recharge, average recharge and maximum recharge calculated based on the historical rainfall data. The recharge scenarios give reduced groundwater quality and high concentration distance from the coast compared to the injection wells. The injection wells inflow rate considered for the scenario is 20 m3/hr and 40 m3/hr. The injection wells inflow rate of 40 m3/hr gives an improved groundwater quality in the coastal wells and also reduction of the concentration distance of the groundwater quality in the coastal river confluence from the coast. In this study, it is found that the current status of groundwater quality is portable. Even though the quality is good, the groundwater quality parameters concentrations are seeming to be increasing, which indicates the vulnerability of quality degradation in the future. In this situation, the groundwater modelling helps us to understand the status of groundwater head and groundwater quality of the study area. Based on the groundwater modelling study, it is found that the injection wells with an inflow rate of 40 m3/hr can improve the groundwater quality of the coastal wells and it also reduces the concentration distance of the groundwater quality in the coastal river confluence.