Risk Assessment Methodology For Groundwater Overdraft Remediations In Cone Of Depression, Jining City, China

In this study, an attempt is made to formulate a new methodology for the first time for qualitative and quantitative groundwater risk reduced assessment (RRA), for risk management and environmental decision making at different levels of groundwater overdraft remediation. In the absence of relevant and adequate data, rather than use one traditional method for groundwater management studies, this new approach envisaged that a combination of two or more methods can relatively give a better understanding of the possible effects of remedial actions in overdrafted aquifer, and significantly limits risk in terms of the results so obtained. Based on elimination procedure similar to optimization techniques, this scenarios-based risk analysis new method couples the scenarios, analytical and probabilistic approaches to assess the potential groundwater risk due to overdraft remedial actions, and it is achieved by coupling Visual MODFLOW with GIS and SUFFER software to evaluate the impact of reduced pumping and artificial recharge on groundwater system qualitatively and quantitatively. It is an extension of the previous efforts.Based on the simplifications of the conceptual model, to represent the physical, chemical and hydrogeological properties of the groundwater system, a computer code Visual MODFLW model was calibrated by trial and error method until a "goodness-of-fit" was achieved between the observed field data and simulated ones i.e. heads and contaminant concentrations for the 1999-2001. Upon verification, it was then used to simulate hydraulic head distribution, contaminant concentrations and water budget in the model domain in order to understand the complex chemical and hydrogeological groundwater system in Jining city. The model is more sensitive to specific yield and recharge than hydraulic conductivity and storage coefficient. An assessment of groundwater overdraft severity by index and overlay techniques using GIS and based on net recharge rate, the initial aquifer saturation thickness, the drawdown, and the pumping rate density shows that the central part which represents about 5.53% of the total area is the most overdrafted area, whereas sections of the western and southern part of the study area are the least overdrafted. The western and southern sections are characterized with high rate of recharge from both precipitation, irrigation fields, and surface water bodies (the Great Canal and Nansi Lake), but low abstraction rate of 0.1954×10~4 m~3/d, while the city centre has the highest abstraction rate of 21.096×10~4 m~3/d .This finding enabled the location and quantification of the most risky area needing urgent remedial action. Artificial recharge by modification of the hydraulic conductivity (K) value of the bed material to 20 m/d, through infiltration and injection wells with a recharge rate of 150 m~3/d and 2000 m~3/d respectively, averagely increases the water level by 6.497 m/a from a decline rate of 2.36 m/a, with an average annual groundwater increased of 3417.69×10~4 m~3/a On the other hand, the quality of water used for recharge purpose greatly determines the recovered groundwater quality which varies spatially in the study area, but generally, the Cl~- concentrations is within the world heath organization standard (WHO) of 200 mg/l (Grade 2 groundwater quality standards, GB/T14848-93 of < 150 mg/l) and the total hardness TH maximum allowable limit of 500 mg/l is surpassed in some cases. An explanation is given herein.Reduction in pumping increases the amount of water in the aquifer storage and water level. In all cases of reduced pumping without artificial recharge (5%, 10%, 15% and 25%), the water level increase rate is less than 2.36 m/a, however in combination with artificial recharge, more than 2.36 m/a groundwater level increase rate is observed, hence addressing the environmental restoration issue with significant water volume stored within the given time limit; in both cases there is no significant difference in increased volume stored for reduced pumping above 15%. The risk due to reduced pumping is more than risk due to reduced pumping with recharge, which is also more than risks due to recharge only with a better water quality. The 22% reduced pumping with artificial recharge seems to be the best risk reduced option as it offers an optimum (trade off) option in terms of both quantity and quality combined for the study area. At this point the annual volume increased stabilized at about 4600 x 104 m~3/a.Further risk assessment based on the ratios of concentrations of Cl~- and TH in recovered water to that of desired limit (200 mg/l) and maximum permissible limit (500 mg/l) for Cl~- and TH respectively shows that, reduced pumping with artificial recharge leads to a spatial higher risk than recharge only especially within the cone of depression, and south-western parts and this can be attributed to the high TH concentrations. On the other hand, in analysing TH using the statistical probabilistic risk assessment approach, there is no significant difference between the risks under the artificial recharge and artificial recharge with reduced pumping, as the probability of exceeding the critical TH concentration of 500 mg/l in both cases is about 0.67; however based on their average values of 546.96 mg/l and 545.16 mg/l respectively, and given the uncertainties and risks associated with probability estimation, one can accept the almost 50% risk for the two scenarios. About 17% and 5.6% of the concentrations observation wells (OB) indicate a probability of more than 0.2 and 0.5 of exceeding the WHO critical TH concentration of 500 mg/l for recharge only scenario respectively. However, for both recharge and reduced pumping, only about 11% and 5.6 % of the OB wells indicate a probability of more than 0.2 and 0.5 respectively of exceeding the WHO critical TH concentration of 500 mg/l.Using the GIS based Natural Break Jenks classification, recharge only has 25% of the study area having low reduced risk (score 0-4), 18.75% having medium risk reduced (score 4-7), and 56.25% having a high risk reduced (score more than 7). For the recharge with reduced pumping, 30 % of the study area having low reduced risk (score 0-4), 23.08 % having medium risk reduced (score 4-7), and 46.15 % having a high risk reduced (score more than 7). Compared with the probability of exceeding allowable limit approach, while the artificial recharge only remedial action stands the best option based on critical limit criteria, the artificial recharge with reduced pumping seems best option based on mean and second quartile probability of exceeding limit criteria. But given the high cost of recharge water treatment, it is only economical and feasible to maintain the recharging water quality within Grade 2 of the groundwater quality standards of GB/T14848-93, and combine recharge with reduced pumping at or about 22%. The proposed South-North water transfer project along the Great Canal can be effectively used for the recharge purpose.The RRA approach takes into consideration both the hydrogeological and hydogeochemical aquifer controlling factors. Generally this research successfully formulated a simple new methodology of quantitative and qualitative risk reduced assessment (RRA) of groundwater remedial options, which can be effectively used for risk management and environmental decision making.