Simultation of Groundwater and Contamination Discharge 23

Simulation of Groundwater and Contamination Discharge from Krishna – Godavari Coast to Bay of Bengal, India

A. Ghosh BobbaEnvironment Canada, National Water Research Institute, Canada Centre for Inland Waters

Burlington, ON, 7R4A6, Canada

Ghosh.bobba@ec.gc.ca

Abstract: The Krishna and Godavari (East and West) districts of Andhra Pradesh, India have a flourishing agriculture and farmers of these districts effectively use the mineral rich alluvium of the Krishna and Godavari deltas. The farmers extensively pump groundwater for irrigation and industries, resulting in excessive drawdown of fresh­water. Also, saltwater intrusion is occurring along coast of these deltas due to over pumping. The nutrients generated by using fertilizers in agriculture are leaching subsur­face as well as are discharging to the coast of Bay of Bengal. Due to higher nutrient discharge, the algal blooms are forming along the coast. By using a numerical model, an attempt is made to determine salinization of subsurface water and to estimate nutrient discharge to beaches. The saline/fresh water interface is investigated on the basis of the geomorphic evolution of the coastal zone and geochemical processes. In this paper, the role of hydrologic processes is investigated through the development, modification, and application of a numerical model for addressing point and non­point source pollution of receiving waters: subsurface water and seepage water to the coast.

IntroductionGroundwater and pollution discharge from deltas to beaches and estuarine waters has been a topic of theoretical and practical interest for at least a quarter century (Valiela et. al., 1992, 1993., Bobba and Singh, 2004). The discharge of groundwater directly into marine waters is called Submarine Groundwater Discharge (SGD). While it is an unseen phenomenon, the influence of submarine groundwater discharge on the ecology of coastal systems may be more important than once thought, due to the potential impacts resulting from contaminants carried in groundwater. This phe­nomenon is being studied by scientists to better understand the interaction

24 A. Ghosh Bobba

of groundwater and surface waters along coastlines (Bobba and Joshi, 1988, 1989). Freshwater eventually moves close enough to shore to meet the denser saltwater that saturates interstitial space in sediments beneath the sea. Salt­water and fresh water are miscible fluids. When water bodies with differing salinities are in contact, molecular diffusion causes mixing across the line of contact. Diffusion coefficients are relatively high in unconsolidated de­posits, and therefore a wide zone of mixing would be encouraged (Bobba 1993a). The effects of diffusion are exacerbated when groundwater and seawater are in motion, and the intergranular structure of the formation causes dispersion and the development of a transition zone due to ground­water and saltwater water movement. External influences such as tides, recharge events and pumping will cause movement of the interface and en­courage mixing, thus increasing the interface thickness. Thus the processes of diffusion and dispersion result in a transition zone where the salinity gradually changes from completely fresh to fully saline, the thickness of the zone depending on these two components. In permeable coastal aquifers which are subjected to heavy abstraction the zone may attain a thickness of up to 100m (Bobba, 1993a). The presence of sea water in the pore space acts together with lower head pressures in the near shore zone compared with offshore to deflect the path of fresh groundwater sharply upward (Bobba, 1993a). As a result, most of the groundwater flow occurs very near the shore. The importance of ground water is not so much because of the magnitude of flow rates, but rather because of the high nutrient concentrations in groundwater compared to those in receiving coastal water. Although highly variable, the nutrient con­tent of groundwater discharging onto coastal water may be up to five or­ders of magnitude larger than concentrations in receiving seawater (Valiela et al. 1992, 1993). Polluted subsurface water is discharged either directly into to the sea, or enters the coastal waters through rivers and by atmos­pheric deposition. The details of point and non­point pollution in coastal watershed were explained by Bobba (2007). The objective of this paper is to apply the numerical groundwater trans­port model to Krishna – Godavari deltas of east coast India to predict the subsurface water pollution and discharge to coast due to agricultural and aquaculture activities in the deltas.

Simultation of Groundwater and Contamination Discharge 25

The Deltas of East Coast IndiaEast coast of Peninsular India is drained by five major river systems: Maha­nadi, Godavari, Krishna, Penner and Cauvery (Figure 1). The geology and climate of Peninsular India are two overriding physical controls affecting the rivers of the region. By effecting the soils and vegetation the geology and climate determine the sedimentological characteristics and the whole process of erosion, transportation and deposition within each drainage ba­sin. The sparse vegetation of the highlands contrasts with the moderately luxuriant vegetation of the river valleys.

The Godavari DeltaThe Godavari is the largest river system in Peninsular India. Arising in the Western Ghats (1620 m AMSL) near Nasik it passes through Maharashtra, Chhattisgarh, Orissa and Andhra Pradesh before joining into the Bay of Bengal near Kakinada on the Andhra coast. There are three main distribu­taries viz: Gautami Godavari, Vasishta Godavari and the Vainateyam. The first two branch at Rajahmundry and then the Vainateyam splits from the Vasishta Godavari at Gannavaram 22 km from the coastline. A major fea­ture of the marginal coastline is the development of the Kakinada sand spit at Neelarevu Point. The spit isolated Kakinada Bay as a shallow inlet which is being infilled by sedimentary contributions from a variety of streams Coringa, Gaderu and Pillavarava. The upper reaches of the Godavari drainage basin is occupied by the Deccan Traps containing the minerals hypersthene, augite, diopside, ensta­tite, magnetite, epidote, biotite, zircon, rutile, apatite and chlorite. The middle part of the basin is principally Archean granites and Dharwars com­posed of phyllites, quartzites, amphiboles and granites. The downstream part of the middle basin is occupied mainly by the Cuddapah and Vindhyan metasediments and and rocks of the Gondwana Group. The Cuddapahs and Vindhyan are quartzites, sandstones, shales, limestones and conglom­erates. The Gondwanas are principally detritals with some thick coal seams. The Eastern Ghats dominate the lower part of the drainage basin and are formed mainly from the Khondalites which include quartz­ feldspar­ gar­net­ sillimanite gneisses, quartzite, calc­granulites and charnockites. In the coastal region the Tertiary Rajahmundry sandstones crop out. The climate of the Godavari drainage basin has high humidity through­out the year effected by the northeast and southwest monsoons. The delta

26 A. Ghosh Bobba

region is semi­arid with an average annual rainfall of 1042 mm and a maxi­mum temperature in May of 37.3oC. The coldest month is January with a mean daily maximum temperature of 26.9oC and a mean daily minimum temperature of 19.2oC.

The Krishna DeltaThe Krishna drainage basin originates near Mahabaleswar (1438 m.) in Maharashtra State within the Western Ghats. Geologically this is the Dhar­war (Karnataka) Craton, which is a Granite­Gneiss­Greenbelt massif. The provenance consists of Deccan Traps and the Dharwar Archean rocks. The river flows east­north­east to the town of Wai and then east­south­east passed Sangli into Karnataka State and then Andhra Pradesh. It crosses

Figure 1. Deltas of East Coast India.

Simultation of Groundwater and Contamination Discharge 27

Peninsular India for about 1280 km before entering the Bay of Bengal. The total drainage length is 25,344 km with a total annual mean runoff of 55764 million cusecs and a maximum – minimum discharge of 33810 – 3 cusecs . The geology of the drainage basin is dominated in the northwest by the Deccan Traps, in the central part by unclassified crystalline rocks and in the east by the Cuddapah Group. The Dharwars (southwest central) and the Vindhian east central) form a significant part of the outcrops within the unclassified crystalline rocks. The deltaic region itself is formed predomi­nantly of Pleistocene to Recent material. The Krishna delta is situated between ~15o 42’ to 16o 30’N and 80o 30’ to 81o 15’E with it’s head at Vijayawada. After cutting the Eastern Ghats the river forms a deltaic plain some 95­km. wide before its four distribu­taries debauch into the Bay of Bengal. The first channel of the river starts near Avanigodda but the three main distributaries of the modern river splits into the Golumuttapaya, Nadimieru and Main channels. A dam at the head controls the flow within the deltaic plain. Vast amounts of mate­rial have been added during the past 50 years at the mouths of the dis­tributaries with the formation of river mouth bars and barrier islands with associated back island lagoons. As the delta pro­graded these lagoons were in filled with finer grained sediments. From Vijayawada to the Bay the average slope is 20 cm/km. The delta itself has an area of ~4736 sq. Km. The Krishna Delta has large tracts of Mangrove Swamps along the coast with maximum concentration surrounding the three main distributaries. Tidal flats occupy a considerable area of the lower deltaic plain especially between the Golumuttapaya and Avanigodda distributaries (Divi island), although the tidal flats may be the product of a degraded inter­distributary bay between two, now abandoned, former channels. Two canyons are rec­ognized off the mouth of the Krishna Delta: Nagarajuna Canyon and Machili Canyon. The climate of the drainage basin is dominated by the southwest mon­soon which provides most of the precipitation for the whole region. High water in the river is August – November and low water is April – May (at Vijayawada). Climatic types range from per­humid through dry sub­humid in the west through semi­arid in the central and eastern parts of the basin. Only in the very south central part of the basin is truly arid. About 75 % of the basin is under a semi­arid climate, receiving monsoonal rainfall. The rainfall of the delta is 910 mm with precipitation mainly in June through October. The temperature is 22 – 27.5 – 34oC (Bobba, 2000).

28 A. Ghosh Bobba

Application of Numerical Model In this work, SUTRA, a finite element groundwater transport model was applied to Krishna­Godavari deltas. The details of the model and applica­tion of the model was explained earlier (Voss, 1984, Bobba, 2000, 1993b, Bobba and Singh, 1995). This model simulates fluid movement and the transport of dissolved substances or energy in the subsurface system. The model uses a two­dimensional, finite­element method to approximate the equations that describe the two interdependent processes being simulated. Either local ­ or regional ­ scale sections having dispersed or relatively sharp transition zones between saltwater and freshwater may be simulated. The results of numerical simulation of saltwater movement show distributions of fluid pressures and dissolved ­ solids concentrations as they vary with time and also show the magnitude and direction of fluid velocities as they vary with time. Almost subsurface properties that are entered into the model may vary in value throughout the simulated section. Sources and boundary conditions may vary with time. The finite ­ element method using quadrilateral elements allows the simulation of irregular areas with irregular mesh spacing. The model has been applied to real field data and observed to give favorable results (Bobba, 1993b, 1998, Bobba, 2002).

Krishna­Godavari Deltas, IndiaThe Krishna­Godavari delta is located in East Coast of India (Figure 1). The details of geology and environmental problems have been explained earlier (Bobba 2002, Bobba, 2000). The deltas lie between the sea level and 12 to 15 ­ m contour. The deltas have a projection of about 35 to 40­km into the sea from the adjoining coast. The deltas consist of alluvial plain. It has a very gentle land slope of about 1­m per km. The coastal line along the study area measures to about 75 km and the general elevation varies from about 2m near the sea to about 13m at the upper reach. Texturally, a major part of the study area consists of sandy loams and sandy clay loams. The silty soils, which are very deep, medium textured with fine loamy soils is located all along the Krishna­Godavari River as a recent river deposits. The very deep, coarse textured soils with sandy sub­soils representing the coastal sand are also found along the sea. The details of the model application to the Goda­vari delta basin have been explained earlier by Bobba (2000, 2002). Figure 2, shows the comparison of observed and simulated hydraulic for Krishna delta and figure 3 shows for Godavari delta. Figures 4 and 5 shows hydrau­

Simultation of Groundwater and Contamination Discharge 29

Figure 2. Observed and computed hydraulic heads of Krishna delta.

Figure 3. Observed and computed hydraulic heads of Godavari delta.

30 A. Ghosh Bobba

lic heads of Godavari and Krishna deltas due to influence of rainy (July– Nov), irrigation for farming and summer (Feb–May) seasons. The predic­tion of water table depth due to irrigation and saltwater intrusion reported earlier (Bobba, 2002). During high tide and irrigation (rainy), the water table is raised. Figures 4 and 5 shows the simulated hydraulic heads of Godavari and Krishna deltas in different environmental conditions due to heavy and long rainy season and drought conditions due to high temperature and high eva­potranspiration. The distance between surface soil and water table in the coastal area is very small, and the material is generally composed of sands, which do not retain significant amounts of moisture under unsaturated conditions. Hence, the water that overflows the soil directly recharges the groundwater. The distance between the water table and surface soil is at a minimum in the central portion of the delta. It has been observed that areas of minimum depth from the ground level to the water table have high fresh­water potential whereas lowering of the water table from the ground sur­face reduces the freshwater potential substantially. The water table eleva­

Figure 4. Simulated hydraulic heads of Godavari delta in dif-ferent seasons (solid line (red) in heavy rainy season, blue broken line (----) drought conditions, solid black line is observed con-tours).

Simultation of Groundwater and Contamination Discharge 31

tion varies from 0.5 m to 1 m from MSL and decreases gradually towards the coastal side. Patches of freshwater zones are also present along coastal areas. In the non­irrigation season and high evapotranspiration in summer months, the water table fallen down to sea level, it may cause an upward movement of saline water in coastal aquifers. The aquifer likely to be saline is more along the eastern side than the southeastern side. Saline water con­tamination due to non­irrigation may be critical to the southern tapering segment of the delta. Higher water table conditions are observed due to more rain and irri­gated water is recharged to the aquifer. The hydraulic gradient is higher in coastal area due to high water levels in the delta. The salt water was flushed out or stopped seawater intrusion to the aquifer. However, if the severe drought conditions (higher temperature, lesser rainfall) occur in the delta, the water table is reduced due to higher evapotranspiration and over pump­ing the ground water for irrigation and domestic purposes. The water table in delta is lower than sea level in along the coast due to over pumping for

Figure 5. Simulated hydraulic heads of Krishna delta in different seasons (solid line (different colors) in heavy rainy season, (broken blue line) – – – – drought conditions and dark solid line, observed contours).

32 A. Ghosh Bobba

agriculture. The hydraulic gradient is towards delta due to that sea water intrusion occurring along the coast. The salt water intruded to the aquifer and freshwater thickness reduced in the delta. Potassium Fertilizers are extensively used for increasing the crop yield. The variation of in potassium concentrations in groundwater of the Goda­vari delta is shown in Figure 6. The peak values are generally observed dur­ing in November. The temporal variation of chloride and bicarbonate from groundwater samples is shown in Figure 7. It is quite clear that the bicarbo­nate concentrations in groundwater are increasing with time.

Figure 6. Variation of Potassium concentra-tion in groundwater in the Godavari Delta (Chachadi and Teresa, 2002).

Figure 7. Variation of [Cl/HCO3+CO3] ratio in groundwater in the Godavari Delta (Chachadi and Teresa, 2002).

Simultation of Groundwater and Contamination Discharge 33

The ratio of chloride/bicarbonate + carbonate can be used as criteria to evaluate sea water intrusion. Chloride is the dominant ion in seawater and it is only available in small quantities in groundwater while bicarbonate, which is available in large quantities in groundwater, occurs only in very small quantities in seawater. Figure 8 shows the relationship with hydraulic head and ground water quality in Krishna Delta. Higher concentrations of total dissolved solids and nitrite was observed in central part of eastern delta (Vyyuru area). Farmers grows sugar cane and rice in that area. The farmers used more ni­trate fertilizers in that part. High nitrate concentrations were observed in that area. High nitrate concentration are observed in shallow wells due to freshwater is floating on top of salt water. The contaminations are not able to transport to deeper formations due to density differences between fresh­water and salt water. Higher nutrient concentrations are discharging to

Figure 8. Relationship between hydraulic head and groundwater quality in Krishna Delta.

34 A. Ghosh Bobba

Krishna River and coast of Bay of Bengal. Figure 9 shows the nutrients plums to discharge to coast and pockets of high nutrient concentrations in delta. The fish farming also increased along the coastal areas. The farmers added fish food in the ponds. Some of the pollutants are infiltrated to sub­surface water. The contaminant plumes are also discharging to coast due to fish farming in coastal areas.

Groundwater Discharge to CoastA major factor that is critical to reasonably estimating annual groundwater flux to the coast using measured unit fluxes is the width of the groundwater outflow face or discharge zone (Figure 10) (the area between the shoreline and offshore). Fresh groundwater discharging to the coast is restricted by heavier saline water that acts as a density barrier, creating an interface be­tween fresh groundwater and saltwater in the coast and preventing dis­

Figure 9. Location of nutrient discharge plumes from Krishna delta to beach.

Simultation of Groundwater and Contamination Discharge 35

charge toward the coast. Fresh groundwater discharge is restricted to a nar­row zone next to the shoreline when the coast contains more saline water. When the coast contains less saline water, and no or low density differences are present, fresh groundwater can discharge in a wider zone to areas fur­ther offshore. To estimate the discharge area to the coast, theoretical widths based on the Dupuit–Ghyben–Herzberg model (Fetter 2001), and aquifer and salinity characteristics are coupled with actual measurements and spa­tial patterns of groundwater flux made as part of our investigation, were used as discussed below. Simple equations derived by Glover (1964) from the Dupuit– Ghyben–Herzberg model of one­dimensional flow in coastal aquifers (Fetter 2001) also can be used to estimate discharge zone width in the saline part of the aquifer. The width of the discharge zone depends on the variables shown in the following equation: (Fetter 2001).

XYo = – Gq

(1 A) 2K

where, XYo = the area of the freshwater – saltwater interface from the shore line (L2) ;

G = rw /(rs – rw ) and rw is the density of freshwater and rs is the density of saline water;

q = discharge from the aquifer at the coastline per unit length of shoreline ( L3 T–1 ) L–1

K = hydraulic conductivity of the aquifer in L T–1

Figure 10. Flow pat-tern near a beach as computed Equation (Glover, 1964).

36 A. Ghosh Bobba

The variable, q, can be computed by approximating the average distance of the freshwater – saltwater interface from the shore (Xo).

q = 2K(XYo)

(1B) G

The variables in Eqs. (1A & 1B) must be known or estimated to calculate an estimate of the width of the discharge zone. Before the discharge per unit of shoreline length (q) can be computed, the dimensions of the aquifer (length and width) must be known to apply the Dupuit–Ghyben–Herz­berg model. The variable G in Eqs. (1A & 1B) is the ratio of freshwater density to the difference between saltwater density and freshwater density (Fetter 2001).

q = 2K(XYo)

G

G = (1.00 g cm–3 / (1.025 g cm–3 – 1.00 g cm–3)

K = 15.354m d–1

(XYo)= 1m2

q = (2x10x1)/40= 0.7675 m3 d–1m–1

Thus, depending on relative densities of freshwater and saltwater at par­ticular locations and times in the coast, the quantities of discharge may change depending on discharge zone.

Calculation of Groundwater Flux by Darcy’s LawThe groundwater flux can be estimated by Darcy’s law also:

Q = KA∂h

(2) ∂x

A is area of the aquifer face discharging to the coast, in m2, based on average length and thickness of the aquifer adjacent to the coast, and hydraulic gra­dient from observed or from numerical modelling data used to calculate discharge to beach.

Q= 15.354*1(0.049) =0.752 m3/day

The computed values by two methods are compared well. The observed data is not available at present.

Simultation of Groundwater and Contamination Discharge 37

Contamination Discharge Along the CoastConcentrations of nitrogen, which occurred primarily as nitrate in samples collected from the wells, with a median concentration of 1.05 mg/L. Using the median nitrate concentration along the coast as the best estimate of groundwater concentration, and assuming a 10­m discharge zone along the coast, we estimated the median flux of nitrogen to the beach to be about 333 tonnes/ year.

ConclusionsThis research has provided a numerical simulation of groundwater flow, coming from the water management of the projected reservoir, on the regional groundwater behaviour on the delta aquifer. A single­phase two­dimensional finite element model, considering open boundary conditions for steep coasts and a sharp interface between freshwater and salt water, was applied steady­state conditions to the phreatic aquifer for fresh water surplus and deficits at the coastline. When recharges of saltwater occur at the coastline, essentially of freshwater deficits, a hypothesis of mixing for the freshwater – saltwater transition zone allows the model to calculate the resulting seawater intrusion in the aquifer. Hence, an adequate treatment and interpretation of the hydrogeological data that are available for the coastal aquifer were of main concern in satisfactorily applying the proposed numerical model. Results of the steady state simulations showed reasonable calculations of the water table levels and the freshwater and saltwater thick­ness, as well as, the extent of the interface and seawater intrusion into the aquifer for the total discharges or recharges along the coastline. As a result of the present hydrogeological simulations on the phreatic aquifer, a con­siderable advance in seawater intrusion would be expected in the coastal aquifer if current rates of groundwater exploitation continue and an impor­tant part of the fresh water from the river is annually channelled from the reservoir for irrigation purposes. The groundwater and contamination dis­charge also calculated from coast to Bay of Bengal.

AcknowledgementsThis paper is dedicated to my mentor and friend, Prof. Lars Bengtsson, University of Lund, Lund, Sweden for his encouragement in research, aca­demic carrier and kindness. The encouragement and support from Patricia

38 A. Ghosh Bobba

Chambers, National Water Research Institute, Environment Canada, Bur­lington, Ontario, Canada is also appreciated. This project supported by National Water Research Institute, Environment Canada, Canada.

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