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number of groundwater models, spatial variations in fluid

density can affect groundwater flow patterns significantly

(Bear et al. 1999; Christensen et al. 2001). For example,

groundwater flow near the coast is often influenced by

density variation, and more complex density-dependent

models are required to simulate the processes such as

saltwater intrusion and submarine groundwater discharges.

Efforts have been made to simulate density-dependentflow with a MODFLOW base code coupled with an

advective and dispersive transport program (Langevin and

Guo 2006). The SEAWAT model uses a combination of

the approaches used by MODFLOW and MT3DMS model

to represent solute transport processes (Guo and Langevin

2002; McDonald and Harbaugh 2003).

This paper discusses a numerical study of saltwater

intrusion in the central Godavari delta region situated

adjacent to the Bay of Bengal coast in East Godavari

District of Andhra Pradesh, India. Significant quantity of

groundwater is withdrawn from the Ravva On-shore Ter-

minal located in the study area. The main aim of this workis to provide useful information that can aid in the pro-

tection of groundwater resources in the study area from

saltwater ingress. A three-dimensional numerical ground-

water flow and solute transport model were developed with

SEAWAT, which draws upon hydrogeological and hyd-

rochemical data collected as part of this study.

Description of the study area

The Godavari delta region is situated in the East Godavari

District of Andhra Pradesh, situated on the east coast of

India. The study region, spread over an area of 295 km2 in

the southern part of Godavari delta, is bounded by the Bay

of Bengal on the east, the Vainateya River on the west, and

alluvial plains on the north (Fig. 1). The exploration wells

for oil are located 1.1 km from the coastline of the Bay of

Bengal, while the Ravva On-shore Terminal wells are

located 0.6 km inland from the coastline. The area has

extensive tidal flats and inlets that receive sea water during

high tides. The area also experiences periodic flooding by

the Godavari River (Gurunadha Rao et al. 2011). Paddy

cultivation and fresh water aquaculture are the major land

uses within the region. The well-distributed Godavari

irrigation canal network acts as a source for irrigation and

drinking water throughout the year. Vasalatippa, Kunava-

ram, and Pikaleru drains are carrying out the irrigation

return flows through the Ravva On-shore Terminal area and

flow into the Bay of Bengal. This 100-year-old canal net-

work contributes significantly to groundwater recharge,

thereby reducing the potential for saltwater intrusion into

shallow aquifers (Chachadi and Teresa 2002; Gurunadha

Rao et al.2011; Naidu et al. 2013).

Geology

The area is underlain by deltaic sediments of early Holo-

cene age with varying proportions of clay, silt, sand, and

gravel with a gentle slope of 0.001 km/km toward the

coast. Groundwater occurs under water table conditions.

However, semi-confined and confined conditions tend to

develop in the area where impervious clay layers overliethe saturated granular zones. Groundwater is being tapped

from shallow open wells with depth range of 38 m as well

as filter point wells penetrating up to 20 m depth. A series

of marine transgression and regression events have greatly

influenced the depositional environments of the delta in the

past. The beach ridges are associated with the delta pro-

gradation (Rengamannar and Pradhan 1991). The study

area includes fluvial landforms such as channels, levees,

back swamps, and geologic floodplains as well as land-

forms influenced by marine processes, such as tidal flats,

beach ridge complexes, and mangrove swamps. The area is

rich in Quaternary alluvial sediments derived from theGodavari River (Rao 1993; Bobba 2002). Since the Qua-

ternary period, the Godavari River has been discharging

large amounts of sediments into the Bay of Bengal, thereby

supporting the delta building processes. The upper delta

region sediments are essentially fluvial, while those in the

lower delta region are fluvio-marine in origin (GSI2006).

The concentrations of iron, manganese, sodium and pH are

increased towards the delta where they approach the mar-

ine environment. The distribution patterns of calcium and

magnesium are mostly controlled by the amounts of shell

fragments and clay minerals, particularly montmorillonite

(Seetaramaswamy and Poornachandra Rao1975).

Hydrogeology

The average annual rainfall in the study area is about

1,137 mm distributed unevenly among an average of 57

rainy days of the year (Gurunadha Rao et al. 2008). About

72 % of the rainfall occurs during the southwest monsoon

season (JuneSeptember), while the rest occur during the

northeast monsoon (OctoberDecember). The area consists

of alluvium with thickness varying from a few meters to

300 m. Clay is present in varying proportions along with

silt and gravel. The alluvium overlies the Rajahmundry

sandstones (CGWB1999). The hydrogeology of the study

area is mainly derived from borehole geophysical logs

collected at Amalapuram, Vodalarevu, and Surasanayanam

villages. Geophysical imaging was carried out with Multi

Electrode Resistivity Tomography (ERT) at 13 different

locations in the Godavari deltaic region, the results of

which indicate that loamy sandy soils are underlain by

thick clay beds of about 3035 m and followed by coarse-

grained sands (Gurunadha Rao et al. 2011, 2013; Lagudu

L. Surinaidu et al.

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et al. 2013). The geophysical logs collected from three

locations at Ravva On-shore Terminal revealed that sandy

clay is underlain by 4555-m-thick clay with fine sand

followed by medium-to-coarse-grained sands up to a depthof 120 m below which clays saturated with saline water are

found up to a depth of 143 m (Gurunadha Rao et al. 2011;

Naidu et al. 2013).

Groundwater occurrence and flow direction

The occurrence and behavior of groundwater are controlled

by topography, soils, climate, geology, and land use of the

area. In the central deltaic region, the groundwater slope is

very gentle with an average hydraulic gradient of 0.3 m/km.

Groundwater levels in the entire Godavari delta fluctuate

significantly in response to recharge and groundwater

withdrawals (CGWB 1999). Forty-two observation wellswere selected to monitor groundwater levels and ground-

water quality in the area. In general, the groundwater levels

near canals and ditches fluctuate 34 m between the pre-

monsoon and post-monsoon season (CGWB1999). How-

ever, during the study period (20062007), they were

observed to be less than 2 m. Maximum groundwater ele-

vation of 5 m above mean sea level (MSL) has been

observed at Amalapuram, while a minimum of -12 m

(MSL) is observed inside the Ravva On-shore Terminal

A

B

AB Profile

Fig. 1 Location map of the

central Godavari delta, East

Godavari District, A.P, India

Assessment of possibility of saltwater intrusion

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wells. The groundwater elevation contours during the pre-

monsoon (2006) period indicate the groundwater flow

direction to be toward the Bay of Bengal coast with a

groundwater gradient of 0.43 m/km from Amalapuram to

coast (Fig.2), and the same trend was observed for

remaining three monitoring periods (Post-monsoon 2006,

Pre- and Post-monsoon 2007).

Materials and methods

For the assessment of groundwater quality, groundwater

samples were collected in the pre- and post-monsoon

periods in 2006 and 2007 from 42 representative dug wells,

bore wells, and hand pumps (filter point wells) distributed

throughout the area (Fig. 1). Samples were analyzed for

pH, electrical conductivity, major ions calcium (Ca2?),

magnesium (Mg2?), sodium (Na?), bicarbonate (HCO3-),

chloride (Cl-), sulfate (SO42-), fluoride (F-), and nitrate

(NO3-) using standard methods recommended by the

American Public Health Association (APHA 2005). The

net groundwater recharge due to monsoon rainfall and

irrigation return flow was estimated using a water level

fluctuation method (GEC 2009) with the following

equation:

Recharge R SyDhArea

where Sy is specific yield and Dh is water level fluctuation

from pre-monsoon to post-monsoon.

SEAWAT, a computer program for simulation of three-

dimensional, variable density, transient groundwater flow

in porous media (Langevin et al. 2003), was constructed

using geophysical data collected during field investigations

in April 2007 by Gurunadha Rao et al. (2011); Naidu et al.

(2013); and Lagudu et al. (2013) and supported with geo-

physical logs collected by Central Ground Water Board

(CGWB), Southern region, India, and Cairn Energy India

Ltd.

N

Amalapuram

Rave terminal

Gudilanka

Vodalarevu

N.Kottaplli

Anantavaram

Fig. 2 Groundwater elevation

contours in m (amsl) in the pre-

monsoon period (2006)

L. Surinaidu et al.

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Major ion chemistry

The pre- and post-monsoon variations in total dissolved

solids (TDS) in the groundwater during the years 2006 and

2007 are presented in Fig. 3. The TDS values of wells C1,

C4, C5, and C6 show very high variations over the 2 years.

Further, the pre-monsoon values tend to be very high in

shallow wells due to seawater infiltration and mixing ofseawater through mudflats. In wells in the Ravva On-shore

Terminal, tapped from greater depth ([67 m), groundwater

is also observed to possess very high salinity.

The mean, minimum, and maximum values for major

ions in groundwater during both pre- and post-monsoon

seasons for the years 2006 and 2007 are shown in Tables 1

and2. In general, the groundwater samples were found to

be brackish in nature with pH varying from 7.5 to 8.9. The

increase in pH during the post-monsoon season suggests

that dissolution has been enhanced due to high interaction

between soil and rainwater as well as due to dilution from

the influx of rainwater of lower alkalinity (Subramanianand Saxena 1983). Total dissolved solids in the pre-mon-

soon season ranged from 256 to 25,088 mgL-1 with an

average value of 5,149 mgL-1, whereas in the post-mon-

soon, the range was from 141 to 28,536 mgL-1 with a

mean value of 6,486 mgL-1. Chloride during the pre-

monsoon ranges from 43 to 13,490 mgL-1 with a mean

value of 2,104 mgL-1, whereas during the post-monsoon,

the mean value increased to 2,943 mgL-1. The seasonal

increase in total dissolved solids and chloride possibly

indicates the dissolution of marine clays by rain water and

irrigation return flows (Gurunadha Rao et al. 2013).

Nitrate, sulfate, and bicarbonate values were found to

increase during the post-monsoon period, which may be

due to the addition of sulfate by organic substances of

weathered soils, sulfate leaching from fertilizers, and other

anthropogenic influences (Kumar et al. 2007). Concentra-

tions of the major cations (Ca2?, Mg2?, Na?, K?) in the

groundwater are very high compared to Bureau of Indian

Standards (BIS) for drinking water in both the pre- and

post-monsoon seasons (BIS2012). The high salinity levels

in the central Godavari delta are attributed to dissolution of

marine sediments and upconing of brines (Gurunadha Rao

et al. 2013; Lagudu et al. 2013).

SEAWAT modeling

The SEAWAT model is a useful tool for simulating various

types of variable density fluid flow through complex

geometries and geological settings, including saltwater

intrusion in coastal aquifers, submarine groundwater dis-

charge, brine transport, and groundwater flow near salt

domes (Ding et al.2014). The fundamental concept of the

SEAWAT model is to combine the two commonly used

groundwater flow and solute transport modeling programs,

MODFLOW (Anderson and Woessner1992; Harbaugh and

McDonald1996) and MT3DMS (Zheng and Wang 1999;

Zheng and Bennett2002) into a single program that solvesthe density-dependent groundwater flow and solute trans-

port equations. The governing equation for density-

dependent groundwater flow in terms of freshwater head as

represented in MODFLOW routines and SEAWAT is well

described by Andersen et al. (1988), Zheng and Bennett

(2002), Langevin et al. (2003), Bakker et al. (2004),

Zimmermann et al. (2006), and Lin et al. (2009).

Model discretization

In this study, the SEAWAT model was applied to Ravva

On-shore Terminal area, in the Godavari Delta region to

simulate three-dimensional, variable density groundwater

flow in porous media. The study area has been divided into

125 rows and 115 columns with a spacing of 200 m. The

aquifer thickness and layer thicknesses were determined by

geophysical investigations at 13 locations and borehole

Cairn Pumping Wells

TDS

(mg/l)

Wells near the coast

Fig. 3 Total dissolved solids

(TDS) in mg/l in the central

Godavari delta in year

(20062007)


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well data at 5 locations (Gurunadha Rao et al.2011; Naidu

et al.2013). A six-layer model was developed to represent

the entire hydrogeological conditions of the study area

using the available borehole geologic logs, largely pro-

vided by Cairn Energy India Ltd. and CGWB. In the

model, layers 1, 3, and 5 represent aquifers, whereas layer

2, 4, and 6 represent confining layers. In the region, there is

no major groundwater fluctuation (Gurunadha Rao et al.

2008), and therefore, the modeling was performed in

steady state with a total 50-year simulation period.

Hydrogeological parameters

Hydraulic conductivity of the groundwater system was

determined through pumping tests conducted in the study

area using Cooper and Jacob (1946) method. Water sam-

ples were also collected for analyses during the pumping

test. Packer tests were also adopted to estimate the

hydraulic properties of different horizons within the aquifer

system (CGWB1999). Average values were computed for

each zone, and the mean hydraulic conductivity value was

assigned to the entire zone. The size of zones was initially

estimated, but then adjusted during model calibration. The

hydraulic conductivity of the sandy layers was determined

to be approximately 10 m day-1 (layers 1, 3, and 5) and

clay layers approximately 1 m day-1 (layers 2, 4, and 6).

The vertical hydraulic conductivity was assumed to be

10 % of the horizontal hydraulic conductivity. The distri-

bution of conductivity values from top to bottom is shown

in Fig.4. The average water level rise in the post-monsoon

of 2006 was 1.07 m. The study area is mostly covered bysandy clay; hence, the specific yield was taken as *0.07

(Sastri et al. 1973; Seetaramaswamy and Poornachandra

Rao 1975). The estimated recharge was 73 mm year-1,

and it is applied only to the upper most active layer. There

is no significant withdrawal of groundwater in the delta

region except from the Cairn Energy India Ltd. pumping

wells in Ravva On-shore Terminal area. The net ground-

water pumping inside the Ravva On-shore Terminal has

been simulated through 5 pumping wells (C30 to C34 in

Table 1 Statistical analysis of

major ions in groundwater,

central Godavari delta, 2006

All values are mg/l except pH

Pre-monsoon 2006 Post-monsoon 2006

Mean Minimum Maximum Mean Minimum Maximum

pH 7.9 7.3 8.9 7.9 7.4 8.8

TDS 8,266 274 27,856 6,307 248 27,771

HCO3-

109 37 220 238 61 1,037

Cl- 1,606 57 6,308 1,665 64 6,996

F- 0.7 0.25 0.95 0.7 0.32 1.02

NO3-N 11 2.2 79.2 3.1 1.1 22

SO42- 88 20 285 97 30 350

Na?

3,753 24 14,260 2,164 32 12,906

K? 247 2 803 163 2 789

Ca2?

156 12 952 210 24 1,864

Mg2?

136 10 596 93 5 803

Table 2 Statistical analysis of

major ions in groundwater,

central Godavari delta, 2007

All values are mg/l except pH

Pre-monsoon 2007 Post-monsoon 2007

Mean Minimum Maximum Mean Minimum Maximum

pH 8.2 7.4 8.9 7.6 6.9 8.2

TDS 5,149 256 25,088 6,486 141 28,536

HCO3- 290 85 610 442 70 2,008

Cl- 2,104 43 13,490 2,943 19 16,221

F- 0.72 0.37 1.06 0.30 0.05 0.74

NO3-N 3 1 18 6 2 47

SO42- 109 30 365 364 11 1,870

Na?

1,276 12 7,898 1,507 6 8,019

K? 67 2 275 61 5 336

Ca2? 254 16 1,224 147 30 1,408

Mg2?

110 5 684 342 4 2,159

L. Surinaidu et al.

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Fig.1) each with a pumping rate of 3,500 m3 day-1 in four

wells. These waters are directly injected into the oil

exploratory wells to realize higher production, while the

water from the fifth well is used for domestic use and

drawing about 4,700 m3 day-1. However, the water from

fifth well has been subjected to reverse osmosis for the use

of drinking and cooking.

Boundary and initial conditions

Because the model is set up to simulate saltwater intrusion

in coastal aquifers, boundary conditions are required for

both groundwater flow and solute transport. Streams and

rivers are specified with the river boundary conditions in

the flow model using the MODFLOW package. Width,

depth, and elevation of the river/drains in the study area

were determined from topographic maps and also from

field observations. Constant-head boundary conditions

were assigned in the model to allow for lateral inflow

across the western boundary near the Amalapuram area and

lateral outflow to the Bay of Bengal with zero head in the

East. The groundwater salinity at different depths was

analyzed by CGWB (1999) and reported as 35,000 mgL-1

below 122 m depth. Therefore, in the bottom-most layer of

the density-dependant solute, transport model starts at an

elevation of-120 m (amsl) with groundwater salinity of

35,000 mgL-1. The zone between -120 and -90 m (amsl)

was given a salt concentration of 30,000 mgL-1 and the

overlying zone a concentration of 20,000 mgL-1 up to an

elevation of-10 m (amsl) near the coast. The intervening

layer with a salt concentration of 10,000 mgL-1 has been

assumed between -60 and -10 m (amsl) for the interior

region of the study area. Along the coast, it has been

assumed with a slightly elevated concentration up to

20,000 mgL-1 for the same layer as per quality analysis.

Up to -10 m (msl) elevation, the salt concentration of

1,000 mgL-1 has been given for interior areas away from

coast based on analytical results of the groundwater sam-

ples at different depths in the study area provided by

CGWB and Cairn Energy India Ltd. as shown in Fig. 5. A

boundary condition constant concentration was specified to

simulate the potential mass flux transport through upconing

phenomena to facilitate the dispersion processes along the

vertical section of the model. The values specified for the

constant concentration boundaries are consistent with the

reported values of initial concentrations of each layer by

CGWB and from the pumping wells of Ravva On-shore

Terminal.

Results and discussions

In this study, groundwater flow model calibration has been

achieved through a trial and error method by adjusting the

two key parameters (i.e., hydraulic conductivity and

recharge rates) until head values as calculated by SEA-

WAT match the observed hydraulic head values to a sat-

isfactory degree. During the model calibration, 42 observed

hydraulic head values measured in 2006 were used. The

aquifer hydraulic conductivity decreased with depth due to

the effect of increased weight of overburden on the aquifer

material (Davis and Deweist1966; De Marsily1986). The

conductivity of the aquiclude (2, 4, and 6 layers) was

reduced to 0.72, 0.61, and 0.55 m day-1 from top to bot-

tom from the initial value of 1 m day-1, whereas con-

ductivity of the aquitard was increased to 11.1, 10.9, and

10.2 m day-1 from 10 m day-1 from top to bottom (1, 3,

and 5 layers). The groundwater recharge was marginally

reduced from 73 to 70 mm year-1. Calibration results

show an overall correlation coefficient of 0.91, root-mean-

square (RMS) error of 1.389 m, standard error of 0.258 m,

and normalized RMS of 8.71 % indicating a reasonable

match between observed and calculated heads (Fig. 6). The

80000 84000 87000 90000 93000 96000 99000 103000

0.5

-30

-60

-90

-120

-150

-180

-200

(m)

(m) (m)

Fig. 4 A typical vertical cross

section of the multilayer aquifer

system and permeability

distribution


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calibrated model also was compared against the 2007 post-

monsoon data. The results did not reflect any significant

change in the root-mean-square or standard errors.

The regional groundwater budget was estimated by

assigning different zones in the study area using a zonebudget package (Harbaugh and McDonald1996; Harbaugh

2005). Results derived from the zone budget indicate that

lateral inflow into the study area was about

5,229 m3 day-1; recharge was about 36,856 m3 day-1, and

groundwater discharge to the Bay of Bengal was about

13,843 m3 day-1. While streams and rivers were receiving

a base flow of 9,146 m3 day-1, the total groundwater dis-

charge through pumping wells that included for oil

exploration and domestic needs was 18,700 m3 day-1. The

difference between total inflow and outflow in the central

Godavari alluvial aquifer was -396 m3 which is the model

mass balance error or model discrepancy that was 0.9 %.

The computed regional groundwater budget clearly showsthat there is a significant groundwater outflow

(13,843 m3 day-1) toward the Bay of Bengal, and hence,

there was no possibility of lateral inflow of sea water into

the delta or the coastal aquifers from the Bay of Bengal.

Further modeling was performed to simulate density-

dependent solute transport modeling using SEAWAT with

2006 pre-monsoon hydrogeological parameters including

TDS values in mgL-1. The computed concentration values

were correlated with observed TDS values, obtained from

35000

30000

20000

10000

Salt (TDS) = 1000 mg/l

(m)80000 84000 87000 90000 93000 96000 99000

0.5

-30

-60

-90

-120

-150

-180

-200

(m)

103000

InactiveFlow

InactiveFlow

Fig. 5 Salt concentrations

(mgL-1) of groundwater at

different depths in the

SEAWAT model, along the

profile AB in Fig.1

Fig. 6 Observed versus calculated heads in the SEAWAT model

Observed Concentration (mgL-1)

CalculatedConcentration(mgL-1)

No. Of Data Points : 28Normalized RMS : 10.2%Correlation Coefficient : 0.914

Fig. 7 Observed versus calculated salt concentrations in the SEA-

WAT model

L. Surinaidu et al.

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the analytical results. During the initial simulations, the

correlation levels were unacceptable. Therefore, greater

effort was made to improve the match by modifying the

magnitude and distribution of the background salt con-

centration closer to the analyzed values. However, the

situation could not be improved much. An attempt was

then made to change the transport parameters: dispersivity

and effective porosity. The dispersivity was changed to

95 m (from 100 m) and effective porosity to 0.17 (from

0.15). With these changes, a good match between observed

and computed TDS concentrations was obtained with acorrelation coefficient of 0.914 and normalized error of

10.28 %. For the concentration calibration, 28 observation

wells were utilized to represent the study area including

four wells in Ravva On-shore Terminal (Fig. 7).

The calculated model was then simulated for the next

50 years using the same hydrological parameters for future

prediction and to assess the extent of saltwater intrusion

due to pumping wells in the deltaic region. Upconing

phenomena were predicted for the years 2011, 2016, 2026,

2036, 2046, and 2056. The simulations indicated that the

phenomena of upconing can extend up to 500 m2 around

the Ravva On-shore Terminal in the year 2056. The cone ofdepression is mainly due to point groundwater withdrawal,

and the groundwater velocity vectors do not show any

lateral flow from Bay of Bengal entering the On-shore

Terminal. Variable density-dependent groundwater flow is

responsible for upconing of salt concentration around the

area due to groundwater withdrawals. The SEAWAT

model shows that the upconing of salt up to a concentration

of about 20,000 mgL-1 has been confined to the layers just

below the Ravva On-shore Terminal. The mixing process

and upconing phenomena in and around pumping wells for

2016 were shown in (Fig. 8).

Comparison of upconing characteristics of the year 2006

with 2016 in the Ravva On-shore Terminal indicates that

200 m2 around the wells could be affected with elevated

salt concentrations. The upconing of salt occurs only in a

very small area confined to in and around the Ravva On-

shore Terminal wells. The pumping wells are tapping

groundwater from a depth [67 m. Further, the aquifer

system separated with insulating clay layer (the second

layer in the model) significantly isolates the vertical flowduring pumping. As a result, the shallow aquifer zone (the

first layer in the model) does not contribute to groundwater

being pumped. Therefore, there was no significant impact

of groundwater pumping on the shallow aquifers around

the pumping wells. This could be seen in typical three-

dimensional cut-away section for the year 2056 (Fig. 9).

The upconing phenomenon is observed to be highly

localized due to excess groundwater pumping at point

locations.

Limitations

The numerical model developed in this study represents the

interpretation of a simplified hydrogeological model

observed in the field. The model synthesizes the current

hydrogeological knowledge prevailing in the region. The

regional aquifer formations are simulated as equivalent

porous media, which is considered reasonable at the given

scale. This knowledge, often qualitative, was translated

into numerical model parameters by a process that involves

various assumptions and simplifications. The vertical

Fig. 8 Upconing phenomena

around groundwater pumping

wells at Ravva On-shore

Terminal after 10 years of

pumping (2016) along the

profile AB in Fig.1


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distribution of aquifer parameters and subsurface concep-

tualization of model have the potential to cause large

errors. These sources of uncertainty have to be suitably

considered when interpreting the modeling results. The

model results should only be used to address questions

pertaining to groundwater flow and its management at a

regional scale. Moreover, the model was calibrated for

steady-state conditions, ignoring transient conditions that

occurred preceding the 50-year simulation period, seasonal

variations, or other density-driven changes in the flow

conditions.

Conclusions

In the Godavari delta, there was no considerable change in

groundwater elevations over the 2 years of observation

period between 2006 and 2007. The groundwater contours

indicate that the groundwater flow is directed toward the

Bay of Bengal with a steep groundwater gradient. The high

salinity in the shallow groundwater is assumed to be theresult of interaction with marine clays and dissolution of

evaporates of early Holocene period. The major ion

chemistry indicates that coastal wells are highly vulnerable

to saltwater intrusion associated with upconing of brines

and mixing of marine waters. The estimated regional

groundwater budget from the model studies indicates that a

significant amount of groundwater discharges as outfall to

the Bay of Bengal. The computed and observed salt con-

centrations from the SEAWAT model do not show

elevated concentrations exceeding 25,000 mgL-1 around

pumping wells in Ravva On-shore Terminal. The presence

of thick impermeable subsurface clay layers regulates the

lateral and vertical flow pattern in the region that can avoid

sea water intrusion during heavy pumping from the deep

aquifer. The large-scale groundwater pumping at point

location was insulated by clay layer and restricted the

upconing of elevated salt concentrations to the close sur-

roundings of the Ravva On-shore Terminal only. The steep

groundwater hydraulic gradients, high hydraulic conduc-

tivity, and the upconing characteristics of the coastal

aquifer system do not permit seawater intrusion despite the

large-scale groundwater withdrawals taking place from

deep aquifers in the region, and the salt concentration was

stable at 25,00028,000 mgL-1 since the beginning of

pumping in 1991. Assuming present hydrological condi-

tions and groundwater pumping, no considerable advance

of saltwater would be expected in the coastal aquifer sys-

tem of the central Godavari delta. Continued groundwater

level and quality monitoring on a regular basis are needed

to verify the efficacy of the models developed and also lead

to a better understanding on process-response characteris-

tics of saltwater mixing in coastal aquifers under heavy

pumping conditions.

Acknowledgments The authors are grateful to the Director, CSIR

National Geophysical Research Institute, Hyderabad, for his kind

permission to publish the paper. We are also thankful to Dr. Paul

Pavelic, International Water Management Institute (IWMI), Hydera-

bad, for his critical inputs and editing the manuscript. The time

provide by IWMI to write this paper is highly appreciated. We thank

Fig. 9 A typical three-dimensional cut-away section of upconing of salt concentration (mgL-1) around Ravva On-shore Terminal wells (NW

SE direction) after 50 years of pumping

L. Surinaidu et al.

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Dr. Virginia Burkett, United States Geological Survey (USGS) for her

useful suggestions and language editing to improve quality of the

paper.

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