Intrinsic vulnerability

INTERNATIONAL JOURNAL and Technology (IJCIET), ISSN 0976 – 6308
International Journal of Civil Engineering OF CIVIL ENGINEERING AND
(Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME
TECHNOLOGY (IJCIET)
ISSN 0976 – 6308 (Print)
ISSN 0976 – 6316(Online)
Volume 3, Issue 2, July- December (2012), pp. 465-476
IJCIET
© IAEME: www.iaeme.com/ijciet.asp
Journal Impact Factor (2012): 3.1861 (Calculated by GISI) © IAEME
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INTRINSIC VULNERABILITY ANALYSIS TO NITRATE
CONTAMINATION: IMPLICATIONS FROM RECHARGE IN FATE
AND TRANSPORT IN SHALLOW GROUNDWATER
(CASE OF MOULARES-REDAYEF MINING BASIN)

Nadia Khelif 1, Imed Ben Slimène 2 and M.Moncef Chalbaoui3
1
(Assitant Professor, Faculty of Science of Gafsa, 2100 Sidi Ahmed Zarroug, Gafsa, Tunisia)
2
(Ph.D, Faculty of Science of Gafsa, 2100 Sidi Ahmed Zarroug, Gafsa, Tunisia)
3
(Hydrogeologist Professor, Institute of Arts and Trades, 9100 Sidi Bouzid, Tunisia)

ABSTRACT

In many rapidly urbanizing cities, groundwaters are constantly affected by
anthropogenic factors such as landscaping, additional abstractions, reduction in catchment
perviousness, etc. Population growth has been uninterrupted and accelerating phenomena in
parts of Moulares-Redayef basin, where urbanization is increasing at an unprecedented rate.
Urban agglomeration is causing radical changes in groundwater recharge and modifying the
existing mechanisms. The Moulares city and majority of the phosphate laundries are sited on
unconfined or semi confined aquifers depend upon wadis water for most of their water supply
and disposal of most of their liquid effluents and solid residues. There has also been an
inevitable rise in waste production. Drainage of surface water has been disrupted as the small
natural channels and low lying areas have been in filled, often with municipal waste.
In agricultural areas, fertilizer application is the main source of nitrate contamination of
groundwater. To develop fertilizer management strategies to combat this problem, arable land
in studied area, the mining basin was evaluated using geographic information system
techniques for intrinsic groundwater vulnerability to nitrate contamination. The DRASTIC
method was modified to adapt it to the Moulares-Redayef environment and used for the
evaluation. The rating for the net recharge factor was also modified to a dilution factor for
contaminants, rather than as a transporter.
However, in the pastures, vulnerability did not exhibit a clear relationship with the frequency
of wells exceeding the standard. This suggests that the modified DRASTIC method is
applicable for fertilizer application management in fields and in the shores of the wadis. In
addition, this method will be useful for deciding the arrangement of arable land taking into
consideration the potential risk of fertilizer-induced nitrate contamination of groundwater.

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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308

Keywords: DRASTIC, Fertilizer application, Groundwater management, GIS Hydrogeology,
Nitrate contamination.

I. INTRODUCTION

Groundwater constitutes an important source of water for domestic, industrial,
agricultural and other purposes. The ever-increasing demand for water due to increasing
demographic pressure has mounted enormous pressure on its judicious utilization. Nitrate
(NO3-) stand for the most well groundwater contaminants globally [1], in the Moulares-
Redayef aquifers and increase-known NO3- concentrations in groundwater in many regions
have been noted. Elevated concentrations of NO3- and increasing concentrations through time
are generally attributed to anthropogenic sources including agricultural fertilizers, septic and
other wastewater sources, livestock facilities, and atmospheric deposition [2][3]. Hence,
several recent studies have noted concentrations above the drinking-water standards outlined
by the US Environmental Protection Agency (10 mg/l as N) [4] in countries like India, China,
Denmark, and the USA [5][6][7][8][9].
The Moulares-Redayef aquifers are quite modest in terms of yield and storage but they have a
proven capability to sustain industrial, domestic and agricultural water supply as well as
provide water for crops that sustain the economy. The exponential increase in the use of this
water resource, has led to widespread aquifer over-exploitation and groundwater quality
deterioration. Therefore there is a need for in-depth understanding of flow and transport
processes in these complex aquifer systems (e.g., quantitative evaluation of the resource,
preservation of the quality, vulnerability assessment).
Despite the likely persistence of elevated nitrate levels in these systems, questions remain
about the impact of irrigation and fertilizer use on groundwater resources.
These aquifers are heavily utilized for mining industries and crop irrigation, large
withdrawals from wells and recharge from irrigation applications can substantially increase
groundwater velocities and vertical flow components [10][11], potentially affecting nitrate
transport and degradation rates. However, the factors controlling the distribution of NO3-
degradation in heterogeneous regional-alluvial-aquifer systems with large pumping
withdrawals are not well understood.
30 sampled wells screened in the Moulares-Redayef aquifer. Nitrate was detected in water
from the majority of wells, with a maximum detection of 103,76 mg/l. Nitrate was more
frequently detected and at higher median concentrations in the alluvium.
Considering the depleting water resources and consequently the mounting problems,
sustainable water resources development plans are needed ([12]Nageswara Rao and Narendra
2006). The policies that control groundwater exploitation are of crucial importance in water
resources management. Towards this, mapping and monitoring of existing groundwater
resources and forecasting the future resource-use scenarios are important.
The integration of information on several environmental features results in zones of
promising groundwater potential in a systematic way and forms an important aspect of
groundwater-management studies. These data, in conjunction with ground truth information,
provide details on geology, geomorphology, structural pattern and recharge conditions, which
ultimately define the groundwater regime. Excessive infiltration of irrigation water can
introduce agricultural contaminants (e.g. nitrate) to shallow groundwater, increase recharge
rates, and significantly alter groundwater residence times [3].
The groundwater prospect/potential maps can show the range in groundwater yield at
different depths, besides indicating probable sites for recharging aquifers. Geographical

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information systems (GIS) provide a means of introducing information and knowledge from
other data sources into the decision-making process and help in handling and management of
large and complex data bases.
GIS facilitates better data analysis and interpretation. [13]Jha and Peiffer (2006) and [14]Jha
et al. (2007) reported pertinent studies on the application of these techniques in the
exploration and assessment of groundwater resources, selection of sites for artificial recharge,
subsurface flow modeling, and assessment of pollution, natural recharge distribution and data
analysis.
The present paper describes the concepts, importance and applicability of GIS technologies in
groundwater studies, and critically reviews the works related to groundwater potential
assessment, to evaluate hydrogeologic factors as explanatory variables for the distribution of
changes in NO3- concentrations over time in a complex regional aquifer system and to assess
the role of recharge dynamics on groundwater flow in a human-impacted sub watershed
based on detailed spatio-temporal field observations.

II. GEOLOGY, SOIL AND HYDROLOGY OF STUDY AREA

The Moulares-Redayef aquifers system occupies much the mining basin, one of the
most economic basins in Tunisia and constitutes largest sedimentary depressions which are
drained by a gathering of wadis system. The surficial geology is characterized by the Plio-
Quaternary sedimentary (alluvial fan deposits) deposits that are surrounded along basin
margins by Miocene bedrocks sands to the northeast and southeast highlighting the
mountains (fig. 1).
This unit does exist where very fine to fine sands generally occur at the surface. However,
shallow aquifers occur at relatively deeper (>15 m below ground level) depths in the central
and southern depression, where the unit is thick.
The composition of soil in different surface geological units of Moulares-Redayef varies as a
function of proportions of sand, loam (silt), and clay.
Average soil composition for individual soil classes was examined and later aggregated over
a total zone setting. Soil composition in major wadis, and Tertiary deposits in eastern and
terrains are predominantly sandy. In contrast, soil compositions in Plioquaternary terraces are
mainly clayey. Surface geology and soil composition which generally characterize shallow
aquifers in Moulares-Redayef basin largely control the timing and pathways of groundwater
recharge to aquifers.
Groundwater flow through high permeability interflow zones within the goundwaters occurs
dominantly between successive flow units (i.e. parallel to stratiform), while localized flow
may occur along vertically oriented fractures and through faults in the center basin
(connection of wadis). Horizontal hydraulic conductivities for the plioquaternary aquifers are
on the order of 2.10-6 à 10–3 m/s with a median value of about 10–6 m/s, while for the
Miocene Aquifer, are on the order of 2.10–4 m/s. Effective porosities for the mining basin
range from than 1% to greater than 3%.
Lateral regional groundwater flow in the aquifers is generally from topographically higher
areas of the Moulares-Redayef Basin headed for the ensemble wadis. Locally, this
generalized flow pattern is complicated by recharge from irrigation water applied to the land
surface, canal leakage, and by discharge of pumping wells. Amongst 1.03 and 5.26 Mm3/year
of industrial and irrigation water is supplied to the studied Basin from drainage ditches
supplied by the Miocene aquifer. The average annual surface-water application rate to the

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study area is 1.22 m/year, and the average application rate of nitrogen fertilizers in the study
area is approximately 15,700 kg N/km2/year.

Fig. 1 Study area and Major surficial geological units

III. SAMPLING AND ANALYSIS

To understand the chemical characteristics of groundwater and the fate and transport
of fertilizer, groundwater samples were collected from 30 wells in Moulares-Redayef Basin.
The collection of groundwater samples was carried out from December 2005. It must be
pointed out that due to the lack of hydrogeological infrastructure at the site, including
piezometers, no point measurements of groundwater quality could be taken. Nearly all the
groundwater sampling wells at the study site are open boreholes. The groundwater sampling
depths range from 4.5 to 60.0 m below ground surface. Be aware that although such a sample
is taken from a point, it may better represent quality of mixed groundwater over a vertical
distance between the water table and the bottom of the wellbore provided that water inside
the open borehole is well mixed.
Water samples were analyzed using ion chromatography to measure nitrate (NO3–), chloride
(Cl–) and sulfate (SO42–) concentrations. Concentrations of sodium (Na+), calcium (Ca2+) and
magnesium (Mg2+) were measured by inductively coupled plasma atomic emission
spectrometry. Bicarbonate (HCO3–) was determined by titration with hydrochloric acid (HCl).
1. Groundwater recharges estimates
Agricultural irrigation water is defined here as groundwater impacted by agricultural
activities in irrigated regions. Excessive infiltration of irrigation water can introduce
agricultural contaminants (e.g. nitrate) to shallow groundwater, increase recharge rates, and
significantly alter groundwater residence times [3]. The rating of net recharge factor in the

468


DRASTIC method Eq (1) was modiﬁed to adapt it to the Moulares-Redayef environment and
used for the evaluation of fertilizer application in agriculture, rather than as a transporter.
However, in the pastures, vulnerability did not exhibit a clear relationship with the frequency
of wells exceeding the standard (fig. 2).

DI=Dr×Dw+Rr×Rw+Ar×Aw+Sr×Sw+Tr×Tw+Ir×Iw+Cr×Cw (1)

where D: depth to groundwater, R: recharge rate (net), A: aquifer media, S: soil media, T:
topography (slope), I: impact of the vadose zone, C: conductivity (hydraulic) of the aquifer r:
rating for the area being evaluated and w: importance weight for the parameter.

Fig. 2 The percentage of area that corresponds to the different vulnerability categories

2. Modified Net recharge Factor
2.1.Description of WetSpass model (Water and Energy Transfer between Soil, Plants, and
Atmosphere in quasi Steady State)
Wetspass is a model developed and integrated by ArcView [15] allows the calculation of
hydrological components such as: potential runoff, interception, infiltration, transpiration,
evaporation from the ground surface and the natural recharge.
This model is based on climate data are the physical parameters of ground such as soil type,
slope, topography, land use, the hydraulic head of groundwater, rainfall, wind speed,
temperature and evapotranspiration potential. The bases of these calculations are [16]:

Evapotranspiration: ET = av ETv +as ETs+ a0 ET0 + ai ETi
   (2)

Water Space: S = a v Sv+ a s S s+ a i S i+ a0 S 0
   (3)

Percolation: R = a v R v+ a s R s +a0 R0 + a i R i
   (4)
The coefficients av, as, a0 and ai are respectively the fractions: plant, soil, water and
impervious areas of a cell raster, and ETv, ETs,ET0, ETi, Sv, S s, S I,, S 0 , R v, R s , R0 and R I
are respectively evapotranspiration, runoff, bare soil, water surface and impervious areas.
Precipitations are taken as a starting point for developing the balance of different component
identified as a raster cell. The water balance of different components is treated as follows:

469


- The Vegetation area: the water balance depend on the seasonal precipitation (P),
Intercepted fraction (I), Blade runoff (Sv), transpiration (Tv) and groundwater recharge (Rv)
and can be calculated using Eq (5) ([17]Batlaan et al;2004):

P =I +S v+ T v+ R v
   (5)

- The Blade runoff: It is calculated based on the amount rainfall intensity. The
interception is considered as the soil infiltration. This term is calculated as follows (Batlaan et
al; 2004):
S v pot = C sv P I (6)
Where; C sv is the runoff coefficient from an area.
The potential runoff is updated with different precipitations’ intensities and according the soil
infiltration capacity [17]:

S = C Hor Sv v - pot  (7)
Where; C Hor is a coefficient from description of the seasonal rainfall contributing to runoff.

- Evapotranspiration: Wetspass deduce the transpiration value from the value of
evapotranspiration potential estimated by Penman formula Eq (8):

T rv = cE0 (8)

Where; T rv is the transpiration from a vegetation area;
E0 is the evaporation potential from a water surface, given by Penman equation;
c is the vegetation coefficient that can be defined as a quotient of the transpiration,
given by Penman-Monteith Eq (9):
γ
1+ ൗ
c= γ (9)
1+ ൘ r
ቂ1+ c ቃ
ra

Where; is the constant of proportionality, is the first derivative of the vapor pressure in
the saturated zone;
γ is the psychometric constant;
rc is the resistance;
ra is the aerodynamic resistance.

2.2.The Recharge estimate
The methodology of the calculation results of the estimation of the spatial distribution of
recharge is taken in the model based on the seasonal variation in the first place; it is possible
to have different levels of depth.
Under natural or pre-developed groundwater-fed irrigation condition, net groundwater
recharge to aquifers can be estimated using Eq (10):

R = Sgw + Qbf + ETgw + (Qgwout - Qgwin) (10)

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Where R is net annual recharge, ∆Sgw is change in groundwater storage, Qbf is base flow to
wadis, ETgw is evapotranspiration from groundwater, and Qgwout - Qgwin is the net
groundwater flow from the study area. ∆Sgw, estimated using the WTF method over long
time intervals (seasonal or annual), is sometimes referred to as “net” recharge [18]. In
Moulres-Redayef basin, Qbf is inhibited during the period when wadis stages are higher than
the water table and the shallow aquifer adjacent to major wadis experiences induced recharge
through bank infiltration. Base flow is restricted to the early part of the dry season (i.e.,
descending limb of the groundwater hydrograph) which does not affect annual water-table
rises. During the monsoon (ascending limb of groundwater hydrograph) soil moisture
sustaining ET is predominantly supplied by rainfall and flood water, and ETgw via capillary
flow is inhibited by direct and indirect recharge fluxes to aquifers. The magnitude of ETgw via
capillary flow during the dry season is unclear. Net groundwater flow (Qgwout- Qgwin ) is
assumed to be negligible throughout the study area due to the absence of substantial hydraulic
gradients in the water table of the shallow aquifer [19] [20].
IV. RESULTS AND DISCUSSIONS
1. Results sampling
Groundwater samples collected showed a minimum difference between the water table and
irrigation wells. Median values of Cl–, Ca, Mg, SO42–, pH, K, Si, Fe, and NO3- are higher in
the water-table well. Water quality in the water-table well may reflect the influence of
agricultural land use on shallow groundwater at this site because Cl−, Ca, Mg, SO42–and
nitrate are commonly applied to the land surface in fertilizer [21] and other soil amendments.
The presence of these applied inorganic constituents at high concentrations near the water
table suggests that there is downward infiltration through the unsaturated zone into the Mio-
plio-quaternary aquifer.
Estimates of groundwater recharge are shown in Figs.3, 4 and 5, for two time periods:
monthly and annual recharge. The results show that actual (net) recharge is higher in
northwestern and western parts of Moulares-Redayef Basin than in eastern parts (Fig. 5). The
monthly average of the groundwater recharge of Moulares-Redayef (Fig. 3), for a period of
eight years, is from to 0.02 to 0.94 million m3 with a maximum of 0.94 in January and a
minimum of 0.02 in July. Greater increases in the net recharge are observed in northwestern.

Fig. 3 Monthly average estimated recharge [22].

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Fig. 4 Annual change of recharges [22].

regions and along the Tabadit wadi; changes in recharge are limited in many area of the
basin. Recent mean annual recharge (1997–2005) is greater than the long-term (2002 to 2005)
mean recharge in some parts of the northwestern Basin.

Fig. 5 Spatial variation map of annual recharge in (2004/2005) with WetSpass [22].

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The estimates of recharge for the period of high waters, revealed a spatial variation of
recharge from West to East (5-3 mm), mainly due to rainfall gradient. The endorheic
depression of Garaaet Ed Douza and bare soils shows low value of recharge, which tend to
zero. In urban areas, it was estimated that the blade refill is 2 to 3 mm, except for the northern
part of the city of Moularès where it is 3 to 4 mm due to the change in soil texture (Fig. 5).
The recharge volume estimated for the year 2004 – 2005, is 2.1 million m3 whether 7.45% of
annual rainfall, 64.8% of this volume is added during the period of high water, and 35.2%
during the period of low water. The total volume of recharge is 568.25 m3/ha/year (Fig. 4).

2. Relation and interactions between groundwater chemistry and recharge
pathways of agricultural irrigation and industrial water
It is shown that the greatest increases in groundwater recharge have occurred where the
density of groundwater-fed irrigation and industrial is highest. Anomalous reductions (−0.5 to
−1 mm/year between 1985 and 2007) in groundwater recharge have taken place in areas of
low groundwater abstraction for irrigation. To assess groundwater chemistry spatial and
temporal variability, a set of measurements of electrical conductivity (EC) of pumping wells
have been collected.
Data show an extreme spatial variability in groundwater chemistry as illustrated by the
average EC map (Fig. 6). An extent zone of higher mineralization can be identified, in the
central part of the study area. Samples were classified in groups based on their position up
gradient or down gradient of the wadis confluence. Higher concentrations in the down
gradient sector of the aquifer cannot be attributed to progressive mineralization along flow
paths because the most down gradient wells are less mineralized. Attribution is given to
sources of higher mineralization likely impact of city sewage, water irrigation and industrial
water. The chemical specificities of ions seem significantly influenced by the recharge as
shown by concentrations observed in November and January, indicating water exchange
between the different zones of the aquifer. The observed correlations between conservative
ions are indicative of mixing of lower and higher concentration zones within each group.
Groundwater chemistry temporal variations as indicated by EC measurements can be
subdivided into two periods: the dry season (April–August) where either stable
concentrations, or the rainfall season (December–March) a progressive increase in
concentrations occur. In most cases dilution occurs due to less mineralized recharge water
(and more especially near the tank), in some cases higher concentrations occurs in the SW
and Central zone with observations in many wells.
Changes in NO3–concentrations over time are primarily related to land use, stratigraphy, and
depth in the aquifer system. Most of the wells having increasing NO3– concentrations are
located on down gradient side of Moulares-Reayef basin in or adjacent to urban land-use
areas. Historical land use maps indicate these areas have been in urban land use for decades,
whereas agricultural land use predominates in the surrounding areas. Reconstructions of
nitrogen fertilizer applications and nitrate concentrations in recharge for the study area
indicate fold increases during 1980–2010. Shallow groundwater beneath urban areas and
agricultural areas in Moulares has higher NO3– concentrations (> 50 mg/l). These wells may
be more strongly influenced by southwestward flowing groundwater with higher NO3–
impacted by agricultural land-use areas to the western of the basin. The presence of
thousands, boreholes backfilled with rock, which trail occasionally storm runoff into
groundwater [23], may also contribute to higher groundwater contaminants concentrations in
the Moulares urban area by increasing the amount of recharge from precipitation, in spite of
diluting concentrations from up gradient land use.

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Fig. 6 Ions distributions (Na+, NO3-, SO42- et Cl-) and CE in the Moulares-Redayef basin [24].

V. CONCLUSION

The ions and pollutants concentrations generate wasted water, in the study area. The
substances released by humans include industrial wastes, domestic sewage, rubbish, organic
and inorganic fertilizers, and pesticides, which include a range of substances that are harmful
to humans. These pollutants transported to surface water in various ways, leading to
deterioration of water quality. The organic pollutants, including COD, BOD, NH3 and NO3
followed sharply increasing trends from subsurface to groundwater. NO3- concentration in
groundwaters of Moulares-Redayef is higher than limit value admissible in some periods.
The range of nitrate concentration is found to vary between 20 and 80 mg/l for 2009–2010.
The detailed field study exploited by a high density of pumping wells (>40) reveals very high
spatial variability in terms of hydraulic parameters (transfer, transmissivity) and groundwater

474


chemistry. This variability can be explained by geological factors and the impact of human
activities. Water irrigation and industrial sewage prompt that most monitoring wells show a
quite rapid hydraulic head increase is combined with an increase in EC in many of them. The
increase in EC may be explained by two different processes or a combination of both: (1)
dissolution by recharge water of salts that were deposited in the topsoil during the dry season
leading to highly mineralized recharge water; (2) an upper limit effect such as two
compartments of the aquifer, the up gradient one being more mineralized, which are
disconnected below a given limit corresponding to the elevation of the deepest connecting
fracture and get connected as the water table rises above this limit during recharge.
Concluding from the analysis results above, most of the regions have a higher constraint of
water environment and are unfit for industry. Water environment pressure along the Tabadit
Wadi is comparatively higher and its water capacity is nearly saturated.
This paper highlights the constraint effects of recharge factor on water environment layout by
integrated evaluation of both sensitivity and pressure of water environment, which is of some
guiding significance in harmonizing the relationship between the industrial and agricultural
development and the water environment bearing capacity. Our results also show that the
evaluation outcome is basically accordant with the actual situation in the study area. Yet, we
should vigorously promote the adjustment of pollution in order to stimulate a sustainable
growth pattern with rapid augmentation, high efficiency, low pollution discharge and low
energy consumption. As for the zoning method, the existing evaluation system still needs to
be improved due to the data access limit. In particular, indices like recharge, wells built-up
and storm rainfall in the evaluation of water environment pressure and bio-diversity in the
evaluation of its sensitivity can be considered in the future study.

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Mais de IAEME Publication

Mais de IAEME Publication (20)

Intrinsic vulnerability