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ISSN: 2157-7587
Hydrology: Current Research
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Investigation of Groundwater Recharge and Stable Isotopic Characteristics of an Alluvial Channel

Modreck Gomo1*, Steyl G2 and van Tonder G3
1Institute for Groundwater Studies, Faculty of Natural and Agricultural Sciences, University of Free State, South Africa
2University of the Free State, Department of Chemistry, Faculty of Natural and Agricultural Sciences PO Box 339, Bloemfontein 9300, South Africa
3University of the Free State, Institute for Groundwater Studies, Faculty of Natural and Agricultural Sciences PO Box 339, Bloemfontein 9300, South Africa
Corresponding Author : Modreck Gomo
Post Doctorate Hydrogeologist Researcher
Institute for Groundwater Studies
Faculty of Natural and Agricultural Sciences
University of Free State, PO Box 339, Bloemfontein 9300, South Africa
Tel:+27747246206, 0514446538 (IGS)
Fax: 051 401 2175 (IGS)
E-mail: [email protected]
Received October 22, 2012; Accepted November 17, 2012; Published November 19, 2012
Citation: Gomo M, Steyl G, van Tonder G (2012) Investigation of Groundwater Recharge and Stable Isotopic Characteristics of an Alluvial Channel. Hydrol Current Res S12:002. doi: 10.4172/2157-7587.S12-002
Copyright: © 2012 Gomo M, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distributon, and reproduction in any medium, provided the original author and source are credited.
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Abstract

An investigation was conducted to assess the recharge of an alluvial channel aquifer that is located in Southern Africa. The investigation utilized stable isotopes, groundwater levels response to rainfall and infiltration tests as complimentary tools to identify and understand groundwater recharge processes of the alluvial channel aquifer. The alluvial channel aquifer is characterized by shallow water table conditions (<3 meters below ground level) and quick groundwater level response to rainfall events thereby justifying the application of the water level fluctuation (WLF) method to estimate recharges rates of the aquifer system. The recharging of the alluvial channel aquifers occurs through high preferential infiltration rates as enhanced by pathways created by tree rooting systems. Cavities and holes created by the burrowing animals also contribute to high infiltrations rates along the riparian zone. Saturated infiltrations rates in excess of 1 m/d were determined on the riparian zone of the alluvial channel aquifer. A groundwater recharge rate of 53 mm/year was determined for the alluvial channel aquifer. The groundwater Oxygen-18 (δ18O) and
deuterium (δ2H) compositions plot below the local meteoric water line (LMWL) indicating that the groundwater in the aquifer was exposed to evaporation prior or during the recharging process. The study shows that groundwater levels response to rainfall events can be used as a qualitative tool to distinguish between piston and preferential recharge mechanisms. In general, the study findings suggest that application of complementary tools to assess groundwater recharge can enhance understanding of the process.

Keywords
Groundwater recharge; Stable isotope; Alluvial channel aquifer; Groundwater levels; Preferential infiltration
Introduction
 
Recharge is an important process through which groundwater in aquifers is replenished by precipitation. Without recharge, the groundwater resources would be completely depleted. An understanding of the groundwater recharge process is therefore important for groundwater reserve assessments and its determination. In general, the sustainable management and protection of groundwater resources requires a comprehensive understanding of the recharge mechanisms and rates.
 
Alluvial river channel aquifers constitute an important source of groundwater for usage in domestic and agricultural activities. By nature alluvial channel aquifers are located along riparian zones where enhanced infiltration [1] can potentially have a huge influence on groundwater recharge processes. High infiltration rates along the riparian zones can be mainly attributed to high vegetation density. Lange et al. [2] found that dense vegetation enhances infiltration rates through preferential pathways created by tree roots.
A number of groundwater related studies have been conducted on alluvial channel aquifers in Southern Africa [3-6]. However, most of the studies have focused on assessment of the potential yield and sustainability of the aquifer with no effort being devoted to enhance the understanding of groundwater recharge processes of the aquifer. Although various techniques exist for groundwater recharge investigation and estimation, it is extremely difficult to assess the accuracy of any method [7] and hence the application of multiple and complimentary methods cannot be overemphasized. This research uses stable isotopes, groundwater levels response to rainfall and infiltration tests as complimentary tools to identify and understand groundwater recharge processes of the alluvial channel aquifer. A comparison was drawn between the groundwater recharge processes of the alluvial channel aquifer to that of the background terrestrial aquifer.
Study Area Setting
The study area is located in the Modder River catchment, downstream of the Krugersdrift Dam in the city of Bloemfontein, Free State Province in South Africa (Figure 1). The study area is surrounded by farms that are mainly characterised by maize crop production. A weir has been built downstream of the Krugersdrift dam for flow measurements. However, it is large enough to impound water for irrigation and recreational fishing.
The study area is generally characterised by arid to semi-arid climate with long periods of low rainfall events. On average, the area receives about 540 mm to 750 mm of precipitation per annum, which is often associated with heavy thunderstorm activities. The riparian vegetation alongside the Modder River banks comprises of tall thorn trees, small Bushveld shrubs and thick grasses. Surface topography slopes towards the Modder River and natural groundwater flow conceptually follows the same direction.
The general geology of the study area is characterised by shale, sandstone and mudstone outcrops of the Beaufort Group located in the Main Karoo Basin. The Main Karoo Basin overlies the central and eastern parts of South Africa. The sediments of the lower part of Beaufort Group (Adelaide Sub-group) within the general area of the study site comprise of unconsolidated quaternary deposits of calcrete, silt-clay and gravel-sands that overlie the shale bedrock. Figure 2 shows patches of calcrete outcrops that were observed in the vicinity of the study area; also shown in the figure is the location of the boreholes that were sampled on the alluvial channel aquifer and background terrestrial aquifer respectively.
At local scale study site consists of a shallow alluvial channel aquifer along the riparian zone and the background aquifer on the terrestrial land. The shallow alluvial aquifer is characterised by unconfined conditions and groundwater. The alluvial channel aquifer and background aquifer are characterised by similar lithology. Boreholes drilled into the aquifers intersected unconsolidated sediments of calcrete, silt-clay, sand-gravel that overlies the shale consolidated formation. However, as shown in table 1 the alluvial aquifer is characterised by shallow groundwater level of less than 3.5 mbgl. The background terrestrial aquifer on the hand has slightly deep groundwater.
There is also a huge difference in the thickness of the overburden unconsolidated sediments between the two aquifers which can have some implications for ground recharge process. Due to its shallow water table conditions, the alluvial aquifer has a thin unsaturated zone, while the zone is thicker for the terrestrial aquifer. In general recharge waters takes less time to reach a shallow water table thus quick recharge can be expected under such conditions. Sand and gravel unconsolidated deposits form the highest yielding hydrogeological unit of the two alluvial aquifer systems because of their naturally high hydraulic conductivity. Groundwater at the site flows from the background aquifer through the alluvial channel towards the river.
Methods and Materials
Sampling of groundwater and analyses
Groundwater and river samples were collected in February and May 2011. Samples collected in February represent the hot and wet season while May is characterized by cold and wet conditions. Groundwater samples were collected from boreholes (Figure 2) located on the alluvial channel aquifer and background terrestrial aquifer.
Groundwater samples from the boreholes were collected after 20 minutes of purging at pumping rate of 0.3 l/s. The purged groundwater samples were collected once the pH, temperature and electrical conductivity (EC) had stabilized. One of the groundwater samples was collected from the water that discharges directly from the aquifer into the river. A bailer was used to collect river samples. The river samples were collected upstream, downstream and adjacent to the alluvial channel aquifer. All samples were collected into 1 liter plastic bottles and refrigerated until analyses. The samples were analyzed for δ2H and δ18O stable isotopic by iThemba Environmental Isotope Laboratory of South Africa.
Water Level Fluctuation (WLF) method
The WLF method (Equation 1) was used to quantify groundwater recharge rates using monitored groundwater level [7]. Groundwater levels were monitored throughout the rain season. The WLF method is based on the assumption and reasoning that increases in groundwater level was only due to groundwater recharge. In other words, any increase in groundwater level implies that recharging waters has reached the aquifer after a rainfall event. The WLF method is therefore suited to shallow aquifers where groundwater level’s respond to rainfall events is often quicker.
                                                                                   (1)
Where, R is groundwater recharge, Sy is the aquifer specific yield, Δh is change in water level and Δt is the time duration when the changes in groundwater levels has occurred.
The WLF method was chosen as the most appropriate method to assess and quantify groundwater recharge of the alluvial channel aquifer based on the following premise:
• The existence of a shallow water table to semi-confined conditions of less than 3 meters below ground level (mbgl).
• There is quick and distinct rises in water level after rainfall events.
• It was evident that groundwater recharge after precipitation was responsible for the increase in groundwater level as compared to other factors such as barometric pressure, aquifer flow exchange and evapotranspiration.
• The method is not affected by the mechanisms through which recharging waters moves through the unsaturated zone. What is more important is the fact that water has reached the water table after a rainfall event as indicated by quick rise of water level.
The nature of the groundwater level respond to rainfall events was also used to distinguish between the recharge mechanisms of the alluvial channel and terrestrial aquifer. Rapid response of groundwater level to rainfall would imply the dominance of preferential recharge mechanism. On the other hand, slow response of groundwater level rise to rainfall events would imply the prevalence of piston/diffuse recharge mechanisms. Groundwater level of the two aquifers was monitored monthly from August 2010 to September 2011. Rainfall records for the study area were collected from South African weather service [8].
Infiltration tests
Infiltration tests were conducted on the riparian zone to assess the potential occurrence of preferential recharge to the alluvial channel aquifer. The tests were conducted within the soil horizon of the riparian zone (0-1 mbgl). Infiltration tests were performed on four sites (Figure 3). Each infiltration site consisted of 3 holes of 30 cm, 60 cm and 100 cm depth respectively. The infiltration holes were designed to test different sections of the soil horizon. In general, the following procedure was used to perform the tests:
1. Identification of test sites and recording of coordinates.
2. Hand auguring of the test holes of 150 mm diameter.
3. Pouring water into the holes and recoding the starting head.
4. Measure the water level (h) drop in the test hole per unit time (t) interval until saturated condition is approached. Steady state conditions are approached when the rate of change of head nears a constant rate.
The Infiltration tests were analyzed based on the inverse auger-hole method [9]. The method is centered on the principle that if an augured uncased hole is filled with water until the soil on its vicinity (below and sideways) is saturated the infiltration rate approaches steady state conditions. The infiltration rate is calculated using Equation 2 [9]. The scatter semi-log plot of (h+½r) against (t) should yield a straight line when steady state conditions have been achieved and the value of K value can be calculated according to Equation 2 with any two pair values of h+½r and t.
                                          (2)
Where: K is the average hydraulic conductivity for the saturated area around the hole (m/s); r is the radius of the hole (m); t is time since start of measuring (s); ht is the height of water column in the hole at time t (m); h0=ht at t=0 (m).
Results and Discussion
Groundwater level response to rainfall
The nature and time of groundwater level respond to rainfall events was used as a qualitative tool to assess and identify recharge mechanisms. Figure 4 shows monthly groundwater level and average rainfall monitored from August 2010 to September 2011. The results indicate rapid groundwater level response to rainfall for the alluvial channel aquifer. Lagged and steady groundwater level response characterises the terrestrial aquifer. In general, the magnitude of the water level rise in the alluvial channel aquifer is high as compared to the terrestrial aquifer.
High rainfall amounts recorded in February and June 2011 were associated with flooding storms and these resulted in rapid rise of the ground water level in the alluvial channel aquifer (Figure 4). However, due to continuous seepage discharge, it only takes a few days before the groundwater levels start the gradual receding. Due to high transmissivity of the gravel-sand deposits and continuous seepage discharge, the recharged groundwater of the alluvial channel aquifer has low residence time. It therefore becomes very difficult to see the effect of low rainfall events on the alluvial channel aquifer groundwater level. Besides meeting the discharge requirements, the recharging waters will also have to fill the pore storage spaces of the alluvial channel deposits. For instance, the low rainfall measured for October, September and November 2010 did not result in any measurable groundwater level increase in the alluvial channel aquifer (Figure 4).
The water levels for the boreholes located in the terrestrial aquifer shows steady rises in response to rainfall as indication of the slow piston recharge process (Figure 4). Due to the thick overburden and low permeability properties (see infiltration rates in section 4.3); the infiltrating water takes long a time to reach the terrestrial aquifer. It therefore implies that the recharging of the terrestrial aquifer is a very slow and prolonged process. The length of the recharge process is further shown by the steady rise of water level that continued from the June 2011 rainfall event to August 2011. During the lengthy recharge process, there is great potential for infiltration fronts from different rainfall events to combine resulting in a continuous gradual rise of groundwater levels as opposed to rapid rise in the alluvial channel aquifer.
It is important to acknowledge the potential contribution of the terrestrial aquifer in recharging the alluvial channel aquifer through aquifer flow exchange during the dry season. The dominance aquifer flow exchange can be noticed by the similar steady rate of groundwater level decrease in the two aquifer systems during the dry season and early periods of the rainfall season (August-December 2010). The same steady rate of groundwater level decrease can be used to infer the occurrence of the similar groundwater flow characteristics between the two aquifer systems during the dry period. It therefore provides further evidence to suggest that through aquifer flow exchange, the terrestrial background aquifer is an important source of recharge to the alluvial channel aquifer during the dry period.
A question can therefore be raised as to which is the main recharge mechanism for the alluvial channel aquifer. During the rainy season, the alluvial channel aquifer is mainly recharged through preferential infiltration from precipitation. However, during the dry seasons, the alluvial channel aquifer receives its recharges through aquifer-inter flow from the terrestrial aquifer. Considering that groundwater is often used for irrigation during dry seasons in farming areas situated along perennial rivers, aquifer-inter flow from the terrestrial aquifer has an important role to sustaining the alluvial channel aquifer. A comprehensive understanding of the two aquifers is therefore needed for groundwater resource development, management and groundwater-surface water interactions.
Stable isotopes
Figure 5 shows the plot of δ2H against δ18O for groundwater and river water for samples collected in February (Feb) and May 2011. The groundwater δ18O values ranges from -4.95 ‰ to -5.25 ‰ with a mean value of -5.09 ‰ and a standard deviation of 0.08 ‰. The groundwater δ2H ranges from -31.28 ‰ to -34.40 ‰ with a mean value of -32.78 ‰ and a standard deviation of 0.78 ‰. The river water δ18O values ranges from -2.70 ‰ to -3.64 ‰ with a mean of -3.12 ‰ and a standard deviation of 0.47 ‰. The river water δ2H ranges from -15.15 ‰ to -17.70 ‰ with a mean value of -16.63 ‰ and a standard deviation of 1.2 ‰. The negative values of δ2H and δ18O indicate depletion of the isotopes relative to the Vienna Standard Mean Ocean Water (VSMOW).
The groundwater δ18O and δ2H plot below but closer to the Global Meteoric Water Line (GMWL) of Craig [10] (Figure 5). The Local Meteoric Water Line (LMWL) for Bloemfontein, or for any other place in Free State Province of South Africa, is yet to be established. The nearest available LMWL is from Pretoria which is situated about 500 km north of Bloemfontein [11]. Although the isotopic compositions of δ18O and δ2H of groundwater from the alluvial channel and terrestrial background aquifer plot close to the GMWL, they all plot far off below the nearest LMWL indicating evaporation effects (Figure 5). It can therefore be inferred that the groundwater in the aquifer was exposed to evaporation effects prior or during the recharging process.
It is also important to highlight that there is a clear distinction in δ2H and δ18O between ground and river water (Figure 5). The river Rainfall Alluvial water is more enriched in δ2H against δ18O implying that it has gone through significant evaporation as compared to water that recharges the aquifers. In February the river water had more enriched δ2H and δ18O isotopic composition than in May. This difference can be attributed to the high evaporation effect on river isotopic composition in February (hot and wet) than in May (wet and cold).
Infiltration rates
Table 2 shows average saturated hydraulic conductivities of the infiltrating water determined on the riparian zone. Sites IH3 and IH4 have higher saturated hydraulic conductivity infiltration rates in excess of 1 m/d. The two sites are located on areas with dense vegetation where organic matter, tree rooting system; cavities and holes created by termites and burrowing animals are potentially the key contributing factors. Low infiltration rates of less than 0.01 m/day were measured on the terrestrial land [12].
The high infiltration rate along the riparian zone is attributed to preferential flow pathways created by dense vegetation tree rooting system, holes and cavities created by burrowing animals. Figure 6 shows some of holes that were created by burrowing animals on the site riparian zone. Assuming that the majority of the infiltrating water reaches the alluvial channel aquifer, preferential infiltration on the riparian zone has great potential to enhance groundwater recharge of the aquifer system. It is important to note that the occurrence of high infiltration rates will also depend on water being able to pond on the surface.
Recharge rates
Tables 3 and 4 shows monthly and total groundwater recharge rates calculated for the boreholes drilled into the alluvial channel aquifer and terrestrial aquifer respectively. The groundwater levels were recorded during the 2010-2011 rainy season (Figure 3). The rising in groundwater were measured in the boreholes after major monthly rainfall events and this justifies the application of the WLF method. A conservative average specific yield value of 0.1 was used based on literature. For unconfined aquifers, specific yield typically ranges from 0.1 to 0.3 [13].
Harmonic mean accumulated recharge rates of 53 and 55 mm/ year were calculated for the alluvial channel aquifer and terrestrial aquifer respectively. Using a total rainfall amount of 680 mm/yr, the groundwater recharge values translate into 8% of the total rainfall/ yr for the two aquifers. Although the aquifers are characterised by different recharge mechanisms, their yearly accumulative recharge values are effectively the same.
The alluvial channel aquifer consists of a shallow water table (<3 mbgl) which implies that recharging waters takes a short time to reach the water table in comparison to the deeper water table of the terrestrial aquifer (>8 mbgl). Preferential infiltration on the riparian zone also contributes to quick recharge of the alluvial channel aquifer. While on the terrestrial aquifer, the recharging water takes longer time to reach the water table due to the thick overlying unconsolidated sediments (Put a conceptual representation of geology between the two aquifer systems. The thick unconsolidated overburden above the terrestrial aquifer water table has the potential to allow accumulation of recharging waters from different rainfall events that can be released later into the aquifer. It can therefore be inferred that the alluvial channel aquifer is characterized by quick-preferential but short lived recharge process while in the terrestrial aquifer the recharge is slow and prolonged. However, the amount of recharge that reaches the two aquifer systems in a year is effectively the same.
Recharge conceptual model
The recharge conceptual model was developed to describe the most likely recharge mechanisms of the alluvial channel aquifer based on the understanding gained from theory and field evidence. Two main different recharge mechanisms can be defined for the alluvial channel aquifer and background terrestrial aquifer respectively. Figure 7 shows the conceptualized recharge mechanisms of the alluvial channel aquifer in relation to the background terrestrial aquifer system.
Preferential infiltration (e) of the accumulated surface runoff (a) on the riparian zone is the major recharge mechanism for the alluvial channel aquifer. Preferential infiltration is facilitated by openings created by tree rooting system. Cavities and holes (b) created by the burrowing animals also contribute to high infiltration rates on the riparian zones. Surface runoff from the terrestrial land also accumulates on the lower riparian zone and thereby enhancing the amount of water available to infiltrate into the shallow alluvial channel aquifer. The high slope of the terrestrial land assists in concentrating the surface runoff and unsaturated zone drainage (d) water towards lower elevated riparian zone. These processes result in overall high recharge rate (g) for the alluvial channel aquifer that leads to rapid groundwater level rise.
Aquifer flow exchange (h) from the terrestrial aquifer is also an important source of recharge for sustaining the alluvial channel aquifer particularly during the dry season. The background terrestrial aquifer mainly recharges through diffuse infiltration (c) in the soil matrix. The terrestrial zone is generally characterized by low infiltration rates as governed by low permeability of calcareous soils and sparse vegetation. Due to the thick overburden, the recharging process of the terrestrial aquifer is generally slow and prolonged.
Conclusions
Several approaches have been shown to give information on the origin of the groundwater that recharges of the alluvial channel. The investigation provided quantitative and qualitative information on the recharge processes and mechanisms of the alluvial channel aquifer. The alluvial channel aquifer is characterized by rapid rise of groundwater levels after rainfall events making the WFL method the most appropriate geohydrological tool for estimating recharges rates of the aquifer system.
The recharging of the alluvial channel aquifers occurs through high preferential infiltration rates as facilitated through openings created by tree roots. Cavities and holes created by the burrowing animals also contribute to potentially high infiltrations rates along the riparian zone. The recharge of the terrestrial aquifer mainly occurs by slow diffuse infiltration through the thick unconsolidated overburden. Harmonic mean accumulated recharge rates of 53 and 55 mm/year were determined for the alluvial channel aquifer and terrestrial aquifer respectively using the WLF method. Using a total rainfall amount of 680 mm/yr, the groundwater recharge values translate into 8 % of the total rainfall/yr for the two aquifers. The study shows that groundwater level response to rainfall events can be used as a qualitative tool to distinguish between piston and preferential recharge mechanisms. In general, the study findings suggest that application of complementary tools to assess groundwater recharge can enhance understanding of the process.
Acknowledgments
We wish to acknowledge the support and funding from the Water Research Commission of South Africa (K5/1760 Bulk Flow and K5/1766 Light Non Aqueous Phase Liquid projects), without them this research would not have been possible. The assistance from the Department of Water Affairs in drilling the boreholes at the test site is also greatly appreciated. Technical field assistance that was offered by Furthermore, we wish to acknowledge the University of the Free State for secondary funding for the analysis of samples and other technical work.
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