Aquifer Unit Management Plan (AUMP) For Groundwater Resources Management of Multi-layered Coastal Aquifer System

To address the vulnerability of coastal aquifer to over exploitation and sea water intrusion, a groundwater resources management plan titled as Aquifer Unit Management Plan (AUMP) is designed to provide valuable guidance for groundwater management of a multi-layered cuddalore coastal aquifer system located east coast of South India. The aquifer unit management plan is based on the summative examination of the aquifer unit disposition, response of hydraulic heads to recharge and discharge and effect of increased pumping on aquifer hydraulics. The safe hydraulic heads and annual exploitable potential were considered as the guiding factors to manage fresh groundwater resources of the coastal aquifer. The results show that the process of sea water intrusion has already started and with the present rate of pumping (1034.86 mcmy -1 ), the sea water intrusion risk has increased manifold all along the coast. The salient management strategies recommended are; restricting pumping to 695 mcmy -1 , stepping up recharge activities in recharge zone, adopting water use eciency methods and regulation in the coastal zone. AUMP is simple and can guide water managers to plan for sustainable groundwater withdrawal and safe guard the coastal aquifer from sea water intrusion and ensure sustainability of groundwater abstraction structures. study of Central Ground Water Board (CGWB) were used to decipher hydrostratigraphic units, delineate major aquifers/aquifer units and develop conceptual model. The hydraulic head data (57 nos) used in the study for analyses are from the groundwater monitoring program of Central Ground Water Board(CGWB 2020). The practical knowledge gained by the authors during eld hydrogeological studies between 2011 and 2014 were applied to arrive at logical assumption wherever necessary.


Introduction
The management of multilayered coastal aquifers are challenging due to its vulnerability to sea water intrusion along the coastal zone. The fragility of the multilayered coastal aquifers and their sensitivity to excessive groundwater withdrawal (Hoori 2020) demands a comprehensive aquifer management plan for holistic understanding of the coastal aquifer system and management of its precious groundwater resources in a sustainable manner(Aristotelis2003, Manivannan and Elango 2019). Studies on groundwater management in coastal aquifers have been carried out by various researchers (Cendrero and (Bear 1999) but management plans that identi es issues and provide less complex strategies to protect fresh groundwater resources is necessary. Due to muddled groundwater development and in absence of groundwater management plan, a multi-layered coastal aquifer (called as Cuddalore Coastal Aquifer System -CCAS) located at east coast of South India is vulnerable to over exploitation and faces threat of sea water intrusion. In lieu of the above, the authors had conceived a management plan named as Aquifer Unit Management Pan (AUMP) that can provide valuable guidance for groundwater management of multi-layered CCAS.

Description Of Study Area And Location
The Cuddalore coastal aquifer system is one of the most proli c coastal aquifer located 200 North of Chennai, India lying between the latitudes 11°30' to 11°55' and longitudes 79°15' to 79°50' and covering an area of 2100 Km 2 . The length (N-S) of the aquifer is 32 kilometer and width (W-E) is 58 kilometer approximately. The northern boundary is de ned by up dip along the Ponnaiyar River and the southern boundary by Vellar River (Fig. 1).
The archeans (Biotite Hornblende Gneiss forms the western boundary and Bay of Bengal Sea constitutes the eastern boundary.Along the west, sandstones crop out and underlie towards the east coast and dip gently eastward beneath the Bay of Bengal. The topographic elevation is at 120 m mean sea level (msl) in the west and gradually slopes towards east where at coast the elevation is at msl. The mean annual precipitation for the years 1999 to 2018 is 1290 mm. This region is intensively irrigated by groundwater with dominant land use consists of cultivated crops (almost 80 % of the area) such as rice, sugarcane, groundnut, banana, cashew, millets and vegetables. Cuddalore and Neyveli and are the prominent towns and Asia's largest open cast lignite mine (Neyveli lignite mine) is operational within the study area since 1956.

Materials And Methods
The Aquifer Unit Management Plan (AUMP) constitutes 7 steps, the description;salient outcome of each step is given as Table 1. An aquifer system constitutes one or many major aquifers and several aquifer units within (CGWB, 2017). For management of coastal aquifers, the dispositions of the aquifer units are to be explored and its hydraulic characteristics are to be assessed. The exploratory drilling data (78 nos), geophysical resistivity logging data (36 nos) generated from the aquifer mapping study of Central Ground Water Board (CGWB) were used to decipher hydrostratigraphic units, delineate major aquifers/aquifer units and develop conceptual model. The hydraulic head data (57 nos) used in the study for analyses are from the groundwater monitoring program of Central Ground Water Board(CGWB 2020). The practical knowledge gained by the authors during eld hydrogeological studies between 2011 and 2014 were applied to arrive at logical assumption wherever necessary.  The CCAS has a layered sequence of aquifers composed of sandstones that thicken and deepen to the east towards the Bay of Bengal Sea. The basement exposed in the western part has not been encountered in the eastern and southwestern part due to thickening of the tertiary sedimentary sequence from west to east (Subramanyam 1969 accounts for varying permeability among the units. The tube wells tapping these units have yield varying between 18 m 3 h − 1 and 220 m 3 h − 1 sustaining for 10 to 12 hours of pumping with 3 to 5 m drawdown.The groundwater ow is from northwest to southeast direction, pre-dominantly horizontal and nally seeps into Bay of Bengal Sea. In region around Neyveli lignite mine, the regional groundwater ow is distorted. Fresh groundwater water discharge through sea bottom occurs asCCASis a part of Cauvery basin extends into the sea for several kilometers (Nagendra and . The estimated recharge rate is 772mcm y − 1 for 1290 mm of rainfall. The natural recharge occurs in all the four aquifer units during monsoon, but recharge to aquifer unit I is comparatively high than the other aquifer units as72 percent of its formation is exposed to the ground surface.

4.3.5
Step IV Assess discharge from the aquifer units The groundwater withdrawal from the aquifer units prescribes the aquifer unit management plan. The discharge from Cuddalore coastal aquifer system occurs as (a) withdrawals by irrigation, industrial, mining activity (depressurization) (b) public supply wells and c) free owing wells (artesian condition). Before 1960, discharge was mainly by pumping from about 145 owing wells to the south of Cuddalore, apart from the under ow towards Gadilam and Vellar rivers (Paul and Subramanyam 1961). The energisation of the dug wells during early 1970's led to increase in groundwater withdrawal for irrigation (Fig. 4). The total annual groundwater withdrawal during early 80's was estimated at 350 to 400 million cubic meters (mcm).
Presently (2019) the estimated annual withdrawal by pumping is estimated at 1034.86 mcm (Table 3) Table 4.  Coastal zone vulnerable to sea water intrusion from coast to 10 km inland. .Numerical modeling of CCAS unveil the region between coast and 10 km inland is highly sensitive even to 5 percent increase in pumping and hydraulic head of the con ned aquifer units at-5 msl reverses the recharge velocity ow direction towards the coastal aquifer or inland which in ideal condition (hydraulic head at msl) should be pointing towards sea (CGWB, 2015).Thus, it is crucial to maintain hydraulic heads (hydraulic equilibrium) of each aquifer units at mean sea level as decrease in hydraulic pressure or negative hydraulic pressure (hydraulic disequilibrium) induced by pumping in any one of the aquifer units initiate reversal of hydraulic gradient. The change in hydraulic head dynamics (from positive hydraulic pressure to negative hydraulic pressure)facilitates the salt water wedge (Holland, 1998) to move inland. To maintain hydraulic equilibrium, equivalent freshwater head that represents the column of fresh groundwater is required to balance the hydraulic pressure at a particular depth and groundwater density (Costall, 2020). The present hydraulic heads of the aquifer units III and IV in the coastal zone ranged between − 3 and − 5 and msl to -2 m respectively. Even though the hydraulic heads of the aquifer units (II, III and IV) exists above the aquifer unit (top layer), hydraulic heads below mean sea level has the potential to trigger salt water wedge to march into the coastal aquifer (Sherif and has occurred up to 9 to 16 km inland and pumping was discontinued as reclaiming the intruded zone is a daunting task.

4.3.8
Step VI Sensitizing public on status of the aquifer units

4.3.9Step VII Aquifer Management Strategies
The hydraulic characteristics of the aquifer units, groundwater development between withdrawal and discharge, response of hydraulic head to recharge and pumping over a period of time and the consequence of increased pumping on existing users were the criteria considered for AUMP.
Numerical modelling which takes into account of the above criteria has guided to develop aquifer management plan (Gnanasundar and Elango, 2000, Elango, 2004, Barazzuoli, 2008.The simulation model results (CGWB, 2015) of existing and proposed groundwater management schemes was used to establish the safe yield, sustainable groundwater exploitation quantity and a reasonable groundwater exploitation scheme for the future. (Table 5). The numerical modeling study carried out by Gnanasundar and Senthilkumar (CGWB 2019a) established that hydraulic heads between ms land-3 m msl has the potential to overcome the hydraulic pressure exerted by salt water front and therefore hydraulic head above msl    Maintaining hydraulic equilibrium of the aquifer units can be achieved by minimizing the percent difference between recharge and discharge (presently 31 %) and operating the aquifer system wherein the present hydraulic heads equals the safe hydraulic head. This is achievable by reducing pumping in a way to match the estimated permissible yield. The permissible yield (Koch 2012) is the annual exploitation potential (E pr ) of the aquifer based on recharge or the total pumping rate that guarantees the average hydraulic head in each aquifer unit does not fall below safe hydraulic head (a vertical distance of -3meters from the mean sea level in the next 10 years). The annual exploitation potential (Er) of the aquifer based on recharge to the aquifer in m 3 / area is calculated using the following formula:  aquifer unit IV can continue to tap drinking water supply to Chennai city ifthe reservoirs feeding Chennai city has poor storage. Also in an event of drought by monsoonal failure drinking water supply in the region can be met from aquifer unit IV.
Based on the guidelines recommended by National Green Tribunal(https://greentribunal.gov.in), declaring coastal zone as'Noti ed area' (from coast to 10 km inland) to regulate pumping wherein registration of existing groundwater abstraction structures, permission to operate wells for drinking and irrigation activity only and restrict new wells are few management strategies that can bring down groundwater withdrawal by ~ 50 mcm/year. In 2020, the National Green Tribunal (NGT) recommended that there must be no general permission for withdrawal of groundwater, particularly to any commercial entity, without an environment impact assessment of such activity and restricted groundwater extraction in over exploited areas for industries except for drinking water(http://cgwb.gov.in/CGWA/NGT-orders.html).Such permission should as per Water Management Plans to be prepared, based on mapping of individual assessment units. The plan strategy to limit pumping by 690 mcmy − 1 with optimum pumping of 1.85 mcm/day and pumping per well as 40 m 3 /day in the coastal zone (maximum well depth 50 m bgl) and 80to 120 m 3 /day in the inland zone is the controllable hydrogeological environment to protect CCAS from sea water intrusion. Monitoring is inclusive part of AUMP; hence specially built piezometers tted with telemetric automatic water level and water quality recorders would enhance monitoring mechanism.Finally, involving stakeholders mainly farmer's would sensitize people to achieve desirable results. Table 5 Aquifer management strategies of Cuddalore coastal aquifer system.

Conclusion
Sustainability of the coastal groundwater resources has been a critical issue as pumping-induced hydraulic head draw down has the potential to reverse the hydraulic gradient into the coastal aquifer. The Aquifer Unit Management Plan (AUMP) is a groundwater resources management plan designed for Cuddalore coastal aquifer system to understand and manage its resources in a sustainable way. AUMP is decisive as it cogitates annual groundwater withdrawal limit, annual recharge, and hydraulic heads of the aquifer units, management strategies and administrative measures. With complex aquifer hydraulics and processes within, aquifer unit wise management interventions can guide water managers to initiate action plan and actswiftly to protect the coastal aquifer system from sea water intrusion.As many coastal aquifers in the sub-continent lack management plan citing its complexity, AUMP designed for Cuddalore aquifer coastal system can be broadly amended to single as well as multi-layered coastal aquifers of the sub-continent.

Declarations
Availability of data and material There are no linked research data sets for this submission. The following reason is given: Data will be made available on request

Competing interests
No competing interests exists.

Funding
The entire study has been carried out as part of annual action plan of NAQUIM studies of the department and there is no funding.

Authors' contributions
Authors contribution both the authors have made equal contribution in the study and also in the preparation of the manuscript. Map showing the location, boundary and regional geology of Cuddalore Coastal Aquifer System, India Conceptual model of Cuddalore coastal aquifer system, South India. History of groundwater withdrawal from the cuddalore coastal aquifer system Long term (Period: 1973 -2018) hydraulic head uctuation and trend of aquifer unit I.