Hydrochemical assessment of groundwater used for irrigation in Rumphi and Karonga districts, Northern Malawi

Physics and Chemistry of the Earth 66 (2013) 51–59

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Hydrochemical assessment of groundwater used for irrigation in Rumphi and Karonga districts, Northern Malawi Elijah M.M. Wanda a,⇑, Lewis C. Gulula a, Ambrose Phiri b a b

Department of Chemistry, Mzuzu University, Private Bag 201, Luwinga, Mzuzu II, Malawi Regional Water Office (N), Mzuzu, Malawi

a r t i c l e

i n f o

Article history: Available online 11 September 2013 Keywords: Kelly’s ratio Permeability index Percentage sodium ion Residual sodium carbonate Sodium adsorption ratio

a b s t r a c t Irrigation water quality is an essential component of sustainable agriculture. Irrigation water quality concerns have often been neglected over concerns of quantity in most irrigation projects in Malawi. In this study, a hydrochemical assessment of groundwater was carried out to characterize, classify groundwater and evaluate its suitability for irrigation use in Karonga and Rumphi districts, Northern Malawi. Groundwater samples were collected during wet (January–April 2011) and dry (July–September 2011) seasons from 107 shallow wells and boreholes drilled for rural water supply using standard sampling procedures. The water samples were analysed for pH, major ions, total dissolved solids and electrical conductivity (EC), using standard methods. Multivariate chemometric (such as Kruskal Wallis test), hydrographical methods (i.e. Piper diagram) and PHREEQC geochemical modelling program were used to characterise the groundwater quality. Electrical conductivity, percentage sodium ion (% Na+), residual sodium carbonate (RSC), total dissolved solids (TDS), sodium adsorption ratio (SAR), Kelly’s ratio (KR) and permeability index (PI) were used to evaluate the suitability of water for irrigation. It was established that groundwater is neutral to alkaline and mostly freshwater (TDS 1000 mg/l) of Ca-HCO 3 type. Groundwater is of low mineralisation which did not show statistically significant variations with respect to depth of shallow wells and boreholes, location and seasonality at 5% significance level. Groundwater from Karonga District was largely oversaturated with respect to both calcite and dolomite, where as that from Rumphi District was undersaturated with respect to both calcite and dolomite. However, the calculated PCO2 values suggested that the groundwater system was open to soil CO2 and that there was possibility of degassing of CO2 during flow, which could increase the pH and subsequently result in the oversaturation of calcite in both districts. Groundwater water samples were stable towards calcite and kaolinite stability field. This suggested that equilibrium of the groundwater with silicates is an important indicator of the hydrogeochemical processes behind groundwater quality in the study area. The calculated values of SAR, KR and % Na+ indicated good and permissible quality of water for irrigation uses. However, samples with doubtful RSC (6% from Karonga district), unsuitable PI (5% and 3% from Karonga and Rumphi, respectively) and a high salinity hazard (56.2% and 20.3% from Karonga and Rumphi, respectively) values restrict the suitability of the groundwater for agricultural purposes, and plants with good salt tolerance should be selected for such groundwaters. A detailed hydro-geochemical investigation and integrated water management is suggested for sustainable development of the water resources for better plant growth, longterm as well as maintaining human health in the study area. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The development of agriculture is a key factor in the socioeconomic growth of developing countries such as Malawi, where agriculture is the main source of sustenance for more than 90% of the population, accounts for more than 90% of its export earnings and contributes 45% to the gross domestic product (Environmental Affairs Department (EAD), 2006). Until recently, more than 90% of the ⇑ Corresponding author. Tel.: +265 881277452; fax: +265 1320568. E-mail address: [email protected] (E.M.M. Wanda). 1474-7065/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pce.2013.09.001

population, which mainly consists of resource poor communities, had been predominantly engaged in subsistence rain-fed agriculture in arid and semi arid areas in Malawi (EAD, 2006). However, due to climate change, rain-fed agriculture is becoming unreliable as a result of erratic rainfall and variable surface water flow in these areas. Exploitation of groundwater has increased greatly, particularly for irrigated agriculture, because it is reliable and readily available to the point of need at relatively cheaper cost. Exploitation of groundwater therefore remains the only option to supplement the ever-increasing demand for irrigation water. However, some of the aquifers are not of adequate usable quality due to

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natural factors or anthropogenic pressures (Drever, 1982; Stallard and Edmond, 1983; Faure, 1998; Epule et al., 2011 and Wanda et al., 2011). Nevertheless, many groundwater supply schemes in developing countries are implemented without necessary attention to quality issues. Irrigation water of poor quality adversely impinges on plant growth (Wilcox, 1948; Richards, 1954; Hem, 1991; Karanth, 1997). The adverse conditions reduce agricultural production, which in turn, reduces agrarian economy thereby adversely affecting sustainable economic development and social prosperity (Subba Rao, 2006; Milovanovic, 2007). Although a number of studies on groundwater quality with respect to irrigation purposes have revealed that qroundwater quality is an important consideration in any hydrochemical assessment of groundwater used for irrigation (Richards, 1954; Buckman and Brady, 1967; Ayers and Westcot, 1985; Western Fertilizer Handbook, 1995; Hanson et al., 1999; Bauder and Brock, 2001; Bauder, 2001; United States Development Agency Natural Resources Conservation Service, 2002; McFarland et al., 2002; Tyagi et al., 2009), irrigation water quality concerns have often been neglected in Malawi’s irrigation projects. Therefore, there is no established link between the hydrogeochemical processes and groundwater quality and hence irrigation water quality in Malawi. It is therefore crucial to establish the current status of groundwater quality and thus its appropriateness for irrigation purposes. In this study, the hydrochemical assessment of groundwater was carried out in predominantly agricultural districts of Rumphi and Karonga district of northern Malawi, where substantial amount of groundwater is being used for irrigation purposes. The purpose of this study was to evaluate and ascertain the suitability of the groundwater for irrigation purposes. A number of researchers have proposed different methods of analyzing irrigation water quality data (Al-Bassam and Al-Rumikhani, 2003; Alobaidy et al., 2010). In this study, total hardness (TH), sodium adsorption ratio (SAR), residual sodium carbonate (RSC), salinity hazard (EC), permeability index (PI), Kelly’s Ratio (KR) and percentage sodium ion (% Na+), regarded as the most effective ways to communicate irrigation water quality (Sinha and Srivastava 1994; Yidana et al., 2008; Zakir et al., 2013) have been used. The results should provide a base for the present and future planning, protection and allocation of usable groundwater supplies for irrigation in Rumphi District, Karonga District and elsewhere.

bedrock mineralogy and structure in the study area produce significant variations in its physical and chemical characteristics. 3. Materials and methods 3.1. Groundwater sampling and analysis In this study, 107 representative sampling shallow wells and boreholes were selected in such a way that they represented different geological formations and anthropogenic activities at varying topography of the study area. Groundwater samples were collected during wet (January–April 2011) and dry (July–September 2011) seasons from shallow wells and boreholes drilled for rural water supply using standard sampling procedures (American Public Health Association (APHA), 2005, International Standards Organisation (ISO), 1993). The shallow wells and boreholes were drilled typically to depths between 18 and 51 m (mean depth = 32.0 ± 2.95 m) in Karonga District and 20–61 m (mean depth = 34.0 ± 0.10 m) in Rumphi District (Table 1 and Appendix 1). Two sets of groundwater samples, each in triplicate, were collected at each sampling point, filtered and stored in new pre-cleaned polyethylene bottles. One set of the samples was acidified with HNO3 (to pH 2) and analysed for major cations, whereas the other set was stored unacidified and analysed for major anions. All samples were analysed for main chemical descriptors using standard methods as follows: Hydrogen ion concentration (pH), electrical conductivity (EC) and total dissolved solids (TDS) were analysed in the field using pre-calibrated portable Hanna model HI-9812 pH-EC-TDS meter (Hanna Instruments Limited). The remaining parameters (Na+, Ca2+, Mg2+, K+,   CO2 3 , HCO3 , Cl ), were determined at of Mzuzu University chemistry Laboratory and Central Water Laboratory. The concentration of Cl was determined titrimetrically. The concentrations of nitrates and sulphates were determined using uv–vis spectrophotometer (Jenway model No. 6405 digital). The total concentrations of Na+, K+, Ca2+ and Mg2+ were determined using flame atomic absorption spectroscopy (Buck Scientific Model 200A). The quality of the chemical data was assessed by computing electrical balance errors by taking the relationship between the total cations (Na+,Ca2+, Mg2+ 2 2  and K+) and the total anions (HCO 3 , CO3 , Cl , SO4 ) for each set of complete analysis of water samples. 3.2. Hydrochemistry of major ions

2. Study area Rumphi and Karonga districts are located in the northern region of Malawi (Fig. 1). Malawi is situated in South East Africa and lies within the western branch of the East African Rift System. The districts have a subtropical climate with a distinct rainy season during November to May. The study area is underlain by the Basement Complex of Precambrian to Lower Palaeozoic low grade metamorphic gneisses (Mafingi group) belonging to amphibolite facies (Carter and Bennett, 1973; Hopkins, 1973; Chilton and SmithCarington, 1984). Overlying these basement rocks, localised to the north western edge of the study area, are Karoo sedimentary rocks (Hopkins, 1973). Alluvial, colluvial and residual deposits (sand, silt, gravel and clays) are widespread and overlay the Precambrian Basement Complex (Hopkins, 1973) (Appendix 1). The general formation sequence of the study area in descending of age order is as follows: sandy soils, alluvial-colluvial sediments, Karoo sediments, weathered Basement, fractured Basement and fresh Basement. The occurrence of groundwater is controlled by a combination of topography and geology. Potential aquifer yield ranges from 0.75 l per second to 3 l per second can be widely obtained where the saturated thickness is more than 10 m. Although the aquifer is relatively extensive, on a local scale, differences in

Kruskal Wallis-test was used to test for variations between Rumphi and Karonga groundwaters. The groundwater samples were classified into groundwater types based on the dominant anions and cations. The composition of the dominant ions was displayed graphically using Piper diagram (Piper, 1944) using GWChart software version 1.18.0. On this diagram the relative concentrations of the major ions were plotted on cation and anion triangles, and then the locations were projected to a point on a quadrilateral representing both cation and anions to give a specific groundwater type. The geochemical modelling program PHREEQC v2.16 (Parkhust and Appelo, 1999), implemented with the MINTEQ.v4 database (Allison et al., 1991), was used to calculate aqueous speciation at the field temperature and the thermodynamic equilibrium conditions of waters with respect to the main mineral phases (calcite and dolomite) present in the aquifer. Saturation indices (SI) were calculated as the log of the ratio between ion activity product and the equilibrium constant. The theoretical partial pressure of CO2, (PCO2), in water of each water sample was also calculated with the same thermodynamic model. The saturation indices were used to indicate whether groundwater was saturated, undersaturated or oversaturated with respect to either calcite or dolomite. The state of saturation of a solution with respect to a

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Fig. 1. Map of Malawi showing Rumphi and Karonga districts.

Table 1 Descriptive statistics of water quality parameters and shallow well/borehole depth and p-values for means of Karonga and Rumphi groundwater. Parameter

pH TDS (mg/l) EC (lS/cm) Magnesium (mg/l) Calcium (mg/l) Sodium (mg/l) Potassium (mg/l) Chlorides (mg/l) Sulphates (mg/l) Bicarbonates (mg/l) Nitrates (mg/l) Carbonates (mg/l) PCO2 (atm) Residual sodium carbonate Permeability index Kelly’s ratio Sodium adsorption ratio % Na+ Shallow well/borehole depth (m)

Karonga district samples

Rumphi district samples

p-Value

Min

Max

Mean

Min

Max

Mean

6.3 50.0 120.0 1.2 3.6 2.4 1.2 4.7 0.0 7.9 0.0 0.0 2.50 23.3 9.7 0.1 0.2 4.5 18.0

8.1 950.0 1735.0 116.6 412.0 166.0 11.6 989.7 505.2 665.7 12.3 72.0 1.21 2.5 156.5 1.0 2.7 40.0 51.0

7.1 ± 0.4 502.5 ± 21.4 962.0 ± 50.0 30.7 ± 1.34 107.5 ± 4.94 42.3 ± 1.70 4.4 ± 1.70 188.7 ± 5.95 28.5 ± 0.00 191.2 ± 15.1 0.6 ± 0.00 25.6 ± 0.00 1.85 ± 0.00 3.9 ± 0.00 50.5 ± 0.10 0.3 ± 0.15 1.0±.00 22.9 ± 0.00 32.0 ± 2.95

6.5 24.0 40.0 1.0 7.4 3.1 1.0 6.0 0.0 25.0 0.0 0.0 3.1 8.8 22.1 0.1 0.2 9.6 20.0

7.8 810.0 1630.0 44.7 142.0 40.0 12.0 398.8 53.0 551.0 3.7 31.0 1.69 0.7 122.1 0.8 1.5 56.2 61.0

7.1 ± 0.41 226.8 ± 27.3 510.4 ± 43.4 11.0 ± 1.04 40.0 ± 6.09 15.3 ± 3.47 4.1 ± 1.01 30.2 ± 4.23 9.0 ± 0.00 144.4 ± 20.0 0.2 ± 0.00 5.2 ± 0.18 2.18 ± 0.00 0.4 ± 0.00 70.5 ± 0.00 0.3 ± 0.10 0.6 ± 0.10 25.1 ± 0.00 34.0 ± 0.10

0.457 0.055 0.054 0.887 0.040 0.128 1.181 0.955 0.130 0.017 0.112 0.201 0.082 0.156 1.713 0.125 0.095 0.098 1.175

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solid was defined as follows: IAP > K (oversaturated); IAP = K (equilibrium, saturated) and IAP K (undersaturated) (Stumm and Morgan, 1996).

X¼

IAP K

ð1Þ

Thus, saturation states of X = 1 indicated equilibrium, X > 1 indicated oversaturation and X 1 indicated undersaturation. The saturation indices of a particular mineral were calculated based on Eq. (2) since PHREEQC corrects the solubility constant K in the equation for ionic strength (Parkhust and Appelo, 1999; Appelo and Postma, 2005):

SI ¼ log X ¼ log

  IAP K

Naþ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi SAR ¼ r ffi

ð3Þ

Ca2þ þMg2þ 2

ðNaþ þ Kþ Þ

#

ðCa2þ þ Mg2þ þ Naþ þ Kþ Þ

 100

ð4Þ

The United States Salinity Laboratory Staff diagram, where SAR is plotted against EC (Richards, 1954) and the Wilcox’s diagram, wherein the EC is plotted against % Na+ (Wilcox, 1955) were used for the classification of groundwater for irrigation purposes. Residual sodium carbonate (RSC) was calculated to assess the suitability of groundwater with respect to high bicarbonate and carbonate content using Eq. (5), where all constituents are in meq/l (Eaton 1950). 2þ RSC ¼ ðHCO3 þ CO2 þ Mg2þ Þ 3 Þ  ðCa

ð5Þ

Soil permeability is affected by long term use of water rich in Na+, Ca2+, Mg2+, and HCO 3 . The permeability index (PI) was calculated using equation 6 (where all concentrations are in meq/l) to assess influence of irrigation water on physical properties of soil.

ðNaþ þ

pffiffiffiffiffiffiffiffiffiffiffiffiffi HCO3 Þ

Ca2þ þ Mg2þ þ Naþ

Ca

þ Mg2þ

ð7Þ

Although all these indices were evaluated in this study, the SAR is probably the only one in current use and is generally considered an effective evaluation index for most water used in irrigated agriculture (Ayers and Westcot, 1985).

4.1. Hydrochemistry

The quality of water used for irrigation is vital for crop yield, maintenance of soil productivity and protection of the environment. The Food and Agriculture Organization (FAO) recommended irrigation water quality determining indices, such as sodium adsorption ratio (SAR), percent sodium ion (% Na+), residual sodium carbonate (RSC), permeability index (PI), Kelly’s ratio (KR) and total dissolved solids, were calculated to evaluate the suitability of the water quality for agricultural purposes (Richards, 1954; Ayers and Westcot, 1985; Al-Bassam and Al-Rumikhani, 2003; Zakir et al., 2013). The SAR was computed using Eq. (3), where the ion concentrations were expressed in meq/l. The % Na+ was computed with respect to relative proportions of major cations present in water, where the concentrations of ions are expressed in meq/l, using Eq. (4)

PI ¼

Naþ 2þ

4. Results and discussion

3.3. Irrigation water quality

"

KR ¼

ð2Þ

A neutral saturation index (SI = 0) meant the groundwater sample was saturated with respect to the mineral and in equilibrium with the solids. A positive saturation index (SI > 0) meant that the groundwater sample was oversaturated with respect to the mineral and precipitation or deposition would occur (Hem, 1991). On the other hand, a negative saturation index (SI < 0) meant the solution was undersaturated with respect to the mineral and dissolution would occur (Drever, 1982; Appelo and Postma, 2005).

%Naþ ¼

Kelly’s ratio was computed to assess suitability of water for irrigation purposes by using Eq. (7) (where all concentrations are in meq/l):

 100

ð6Þ

Analytical results of chemical analysis and the statistical parameters such as minimum, maximum and mean for Karonga and Rumphi groundwater samples are presented in Table 1. The calculated electrical balance errors were found to be 610% which is an acceptable error. This meant that the results were reliable. The results of the Kruskal Wallis test (Table 1) revealed that there were no any statistically significant differences between the means of Rumphi and Karonga data sets at 5% significance level. Similarly, the results of the Kruskal Wallis test revealed that there were no statistically significant seasonal variations between the means of dry and wet season data for both Rumphi and Karonga data sets at 5% significance level. It was therefore considered that results from the dry season are representative of the water quality in the study area. In this study pH range was 6.30–8.10 for Karonga District (mean pH = 7.1 ± 0.4) and 6.53–7.80 for Rumphi District (mean pH = 7.1 ± 0.41). This implied that the groundwater of the study area is neutral to alkaline in nature. It was observed that EC ranged from 120 to 1730 lS/cm in Karonga district (mean EC = 962.0 ± 21.4 lS/cm) and 40–1630 in Rumphi district (mean EC = 510.4 ± 43.4 lS/cm). The corresponding TDS ranged between 50 and 950 mg/l in Karonga district (Mean TDS = 156.41 mg/l) and 24–810 mg/l in Rumphi district (mean TDS = 315 mg/l). All groundwater samples could be classified as fresh (TDS < 1000 mg L1) based on Fetter (1990) TDS classification. Groundwater from Karonga District was observed to be more mineralised than that from Rumphi District. Based on topographic patterns, Rumphi District lies in the recharge zone and Karonga District lies in the discharge zone of the study area. Overall, EC and the corresponding TDS did not show any pattern with depth and location of shallow wells and boreholes. The anion chemistry of the analysed samples shows that HCO 3, are the most dominant ions in both Karonga and Rumphi districts. It was observed that HCO 3 ranged from 7.9–665.2 mg/l in Karonga district (mean HCO ¼ 191:2  15:1 mg=l) and 25.0–510.0 mg/l in 3 Rumphi district (mean HCO 3 ¼ 144:4  20:0 mg=l). The ranges of Cl were 4.7–989.7 mg/l in Karonga District (mean Cl = 188.7 ± 5.95 mg/l) and 6.0–398.8 mg/l in Rumphi District (mean Cl = 30.2 ± 4.23 mg/l). The ranges of CO32- were 0.0– 72.0 mg/l in Karonga district (mean CO2 3 ¼ 25:6  0:00) and 0.0–31.0 mg/l in Rumphi district (mean CO2 3 ¼ 5:2  0:18 mg=l). Nitrates ranged from 0.0 to 12.3 mg/l in Karonga District (mean NO 3 ¼ 0:6  0:00 mg=l) and 0.0–3.7 mg/l in Rumphi District (mean NO 3 ¼ 0:2  0:00 mg=l). The ranges of sulphates were 0.0– 505.2 mg/l in Karonga District (mean SO2 4 ¼ 28:5  0:00 mg=l) and 0.0–53.0 mg/l in Rumphi District (mean SO2 4 ¼ 9:0  0:00 mg=l). The order of anionic abundance (in meq/ 2 2   l) in the study area is HCO 3 > Cl > SO4 > CO3 > NO3 . The cationic chemistry was largely dominated by calcium in both Karonga and Rumphi districts. The ranges of Ca2+ were 3.6– 412.0 mg/l in Karonga District (mean Ca2+ = 107.5 ± 4.94 mg/l)

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and 7.4–142.0 mg/l in Rumphi District (mean Ca2+ = 40.0 ± 6.09 mg/ l). The ranges of sodium were 2.4–166.0 mg/l in Karonga District (mean Na+ = 42.3 ± 1.70 mg/l) and 3.1–40 mg/l in Rumphi District (mean Na+ = 15.3 ± 3.47 mg/l). The ranges of Mg2+ were 1.2– 116.0 mg/l in Karonga District (mean Mg2+ = 30.7 ± 1.34 mg/l) and 1.0–44.7 mg/l in Rumphi District (mean Mg2+ = 11.0 ± 1.04 mg/l). The ranges of potasium were 1.2–11.6 mg/l in Karonga District (mean K+ = 4.4 ± 1.70 mg/l) and 1.0–12.0 mg/l in Rumphi District (mean K+ = 4.1 ± 1.01 mg/l). The order of cationic abundance (in meq/l) in the study area is Ca2+ > Na+ > Mg2+ > K+. A graphical display of major ion composition of groundwater using Piper diagrams (Fig. 2) indicated that the majority of groundwater samples from Rumphi district (82%) (Fig. 2a) were characterised by cationic composition dominated by Ca2+ with their anionic composition dominated by bicarbonates. Thus their median chemical composition is characterised by Ca–HCO3 water type. In addition groundwater samples from Rumphi district also contain some mixed cation-HCO3 (12%) and (Na + K)–HCO3 (6%) water types. Similarly, the majority of groundwater samples from Karonga district (78%) (Fig. 2b) have their median chemical composition characterised by Ca–HCO3 water type. In addition, 10%, 7% and 5% of groundwater samples from Karonga district also contain some mixed cation-HCO3, mixed cation-Cl, and Ca–(Na + K)–HCO3 water types, respectively. The saturation indices of calcite and dolomite (Appendix 1), calculated using PHREEQC v2.16 geochemical modelling software, were used to determine the chemical equilibrium between these minerals and water. The solubility of calcite (Eq. (8)) and dolomite (Eq. (9)) is mainly influenced by CO2 fugacity and pH, according to the reactions (8) and (9):

CaCO3 ðsÞ þ H2 OðlÞ þ CO2 ðgÞ ¼ Ca2þ ðaqÞ þ 2HCO3 ðaqÞ

ð8Þ

Calcite ðCa1x Mgx ÞCO2 þ CO2 ðgÞ þ H2 O ¼ ð1  xÞCa2þ þ xMg2þ þ 2HCO3 ð9Þ Dolomite The saturation indices revealed that a majority of samples from Karonga District were oversaturated with respect to both calcite (72%) and dolomite (64%) (Appendix 1). On the other hand, only

Fig. 2a. Piper plot for Rumphi groundwater samples.

55

Fig. 2b. Piper plot for Karonga groundwater samples.

28% and 36% of Karonga District samples were undersaturated with respect to calcite and dolomite, respectively. This meant that the majority of Karonga District samples would precipitate (deposit) more of dolomite and calcite. However, a majority of samples from Rumphi District were undersaturated with respect to both calcite (65%) and dolomite (67%), whereas, 35% and 33% of Rumphi District samples were oversaturated with respect to calcite and dolomite, respectively. This meant that a majority of Rumphi District samples would take more dolomite and calcite into the solution. However, the PCO2 value varied from 2.50 to 1.2 atm (mean PCO2 = 1.95 atm) in Karonga District and 3.08 to 1.69 atm (mean PCO2 = 2.18 atm) in Rumphi District, which were much higher than the atmospheric PCO2 (3.5 atm) (Table 1). Such elevated values of PCO2 suggested that the groundwater system is open to soil CO2. Since the PCO2 values in soil zones are very high, there is possibility of degassing of CO2 during flow, which can increase the pH and subsequently result in the oversaturation of calcite (Singh et al., 2011). The major component activities calculated using PHREEQC v2.16 geochemical modelling software were plotted on a stability field diagram (log(aCa2+/a2H+) vs. log(aH4SiO4)) of the CaO–SiO2–Al2O3–H2O systems at 25 °C (Fig. 3). It was observed that groundwater water from both Rumphi and Karonga districts plotted essentially in the calcite and kaolinite

Fig. 3. Activity diagram for log(aCa2+/a2H+) vs. log(aH4SiO4) for Karonga and Rumphi groundwater samples at 25 °C.

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stability fields. This suggested that equilibrium of the groundwater with silicates is an important indicator of the hydrogeochemical processes behind groundwater quality in the study area. 4.2. Irrigation water quality According to the FAO Irrigation and Drainage Paper No. 20, parameters such as electrical conductivity (EC), percentage sodium ion (% Na+), residual sodium carbonate (RSC), total dissolved solids (TDS), sodium adsorption ratio (SAR), Kelly’s ratio (KR) and permeability index (PI) are important in assessing the suitability of water for irrigation (Ayers and Westcot, 1985). Regarding to the TDS content the water is considered satisfactory when it contains lesser than 1000 mg/l, fair if it contains between 1000 and 2000 mg/l, and inferior when it exceeds 2000 mg/l. Based on TDS groundwater from all sampled sites in the study area was considered satisfactory and suitable for irrigation purposes. 4.2.1. Residual sodium carbonate (RSC) The excess sum of carbonate and bicarbonate in groundwater over the sum of calcium and magnesium also influences the unsuitability for irrigation. According to the Richards (1954), RSC value less than 1.25 meq/l is good for irrigation, a value between 1.25 and 2.5 meq/l is of doubtful quality and a value more than 2.5 meq/l is unsuitable for irrigation. The water with high RSC has high pH. Therefore, land irrigated with such water becomes infertile owing to deposition of sodium carbonate (Eaton, 1950). Hence, continued usage of high RSC waters will affect the yields of crop. In this study, RSC ranges were 23.3 to 2.5 meq/l in Karonga District (mean RSC = 3.9 meq/l) and 8.8 to 0.7 meq/l in Rumphi District (mean RSC = 0.4 meq/l). All groundwater samples from Rumphi District were considered safe for irrigation purposes (RSC < 1.25 meq/l). The majority of groundwater samples from Karonga District (94%) were considered safe for irrigation (RSC < 1.25 meq/l) with the remaining (6%) considered to be within the maginal range (RSC range = 1.25–2.5 meq/l) (Ghosh et al., 1983). Further, the value of RSC is negative at the majority of sampling sites in both Karonga (mean RSC = 3.9 meq/l) and Rumphi districts (mean RSC = 0.4 meq/l), indicating that there is no complete precipitation of calcium and magnesium in the study area (Tiwari and Manzoor, 1988). 4.2.2. Permeability index (PI) The soil permeability is affected by the long-term use of irrigated . water rich in Na+, Ca2+, Mg2+, and HCO 3 and the soil type. Doneen (1964) classified irrigation waters in three PI classes. Class-I and Class-II water types are suitable for irrigation with 75% or more of maximum permeability, while Class-III types of water, with 25% of maximum permeability, are unsuitable for irrigation. In this study, 75% of the groundwater samples from Karonga district and 78% of groundwater samples from Rumphi District fall in Class-I and 20% of Karonga groundwaters and 19% of groundwater samples from Rumphi fall in Class-II, implying that the majority of the groundwater is good for irrigation usage (Domenico and Schwartz, 1990). However, 5% of the water samples collected from Karonga District and 3% of groundwater samples from Rumphi District belong to Class-III, the unsuitable category. 4.2.3. Kelly’s ratio (KR) The Kelly’s ratio of unity or less than one is indicative of good quality of water for irrigation whereas above one is suggestive of unsuitability for agricultural purpose due to alkali hazards (Karanth, 1987). It was observed that the Kelly’s ratio for all the groundwater samples in the study area is below the unity (mean KR = 0.03 ± 0.15 for Karonga groundwater samples) and (mean KR = 0.03 ± 0.10 for Rumphi groundwater samples) (Table 1). This

suggested that, all the samples from study area are good for irrigation with respect to alkali hazards. 4.2.4. Sodium absorption ratio (SAR), percent sodium ion (% Na+) and electrical conductivity (EC) Electrical conductivity is a good measure of salinity hazard to crops, as it reflects the TDS in groundwater. High salt content in irrigation water causes an increase in soil solution osmotic pressure (Thorne and Peterson, 1954). The osmotic pressure is proportional to the salt content or salinity hazard. The salts, affecting the growth of plants directly, also affect the soil structure, permeability and aeration, which indirectly affect plant growth. Electrical conductivity is thus the most influential water quality guideline on crop productivity. Higher EC indicates that less water is available to plants. Based on EC values, 14.1%, 29.7% and 56.2% of groundwater from Karonga District could be classified as excellent (C1, EC range = 100–250 lS/cm), good (C2, EC range = 250–750 lS/ cm) and doubtful (C3, EC = 750–2,250 lS/cm) for irrigation, respectively. Similarly, 36.7%, 43.0% and 20.3% of groundwater from Rumphi District could be classified as excellent, good and doubtful; for irrigation, respectively. Groundwater samples falling in the doubtful class or high-salinity hazard class (C3) may have detrimental effects on sensitive crops and adverse effects on many plants. Such areas require careful management practices of salinity control and selection of plants with good salt tolerance. Sodium concentration is important in classifying irrigation water because sodium reacts with soil to reduce its permeability. Sodium content is usually expressed in terms of percentage sodium or soluble sodium percentage (% Na). The % Na+ in the study area ranged between 4.5% and 40.0% for Karonga District and 9.6– 56.2% for Rumphi District (Table 1). The % Na+ values reflected that the water was under the category of excellent to good irrigation water class (% Na+ < 60%) (Wilcox, 1948). Such irrigation water cannot cause soil aggregates to disperse and subsequent reduction in its permeability hence very suitable for irrigation. Sodium adsorption ratio varied from 0.16 to 2.66 in Karonga district and 0.20–1.30 in Rumphi district. Richards (1954) and Todd (1980) classified irrigation water with SAR values less than 10 as excellent (S1 class) and the water is evaluated suitable for any crop and can be used for irrigation in almost all types of soils. On the basis of the US salinity diagram (Fig. 4), in which EC is taken as salinity hazard and SAR is taken as alkalinity hazard (Richards, 1954), 14.1% and 36.7% of the Karonga and Rumphi groundwaters, respectively belonged to C1–S1 (EC < 250 lS/cm; SAR < 10), a low salinity hazard and low sodium hazard groundwater category that could be used on most soils with little likelihood that soil salinity would develop and with little danger for the

Fig. 4. United States Salinity Laboratory Staff (USSLS) used to classify groundwater in terms of degree of suitability for irrigation: a plot of EC vs. SAR, after Richards (1954).

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Fig. 5. Wilcox diagrams used to classify groundwater in terms of degree of suitability for irrigation: a plot of EC vs. % Na+, after Wilcox (1955).

development of harmful levels of exchangeable sodium (Richards, 1954). Further, 29.7% and 43.0% of the Karonga and Rumphi groundwaters, respectively belonged to C2–S1 (EC = 250–750 lS/ cm; SAR < 10), a medium salinity hazard and low sodium hazard groundwater category that could be used if moderate amount of leaching occurs and with little danger for the development of harmful levels of exchangeable sodium. Plants with moderate salt tolerance such as millet, maize, tomato, cabbage, potatoes, onions, mangoes, bananas, pear, apples and citrus fruits (Sharma and Chawla, 1977) could be grown in C2–S1 groundwater in most cases without special salinity practices for salinity control (Richards, 1954). Furthermore, 56.2% and 20.3% of the Karonga and Rumphi groundwaters , respectively belonged to C3–S1 (EC = 750– 2500 lS/cm; SAR < 10), a high salinity hazard and low sodium hazard groundwater category that cannot be used on soils with restricted drainage although they have little danger for the

Table A.1 Depth, log activities of Ca, H and H4SiO4, saturation indices and major formations of all sampled sites in Karonga and Rumphi districts. Site No.

Log activity Depth (m)

Karonga district sampled sites 1 28 2 35 3 18 4 33 5 22 6 44 7 30 8 22 9 32 10 25 11 42 12 28 13 28 14 39 15 30 16 31 17 45 18 43 19 30 20 41 21 34 22 51 23 41 24 41 25 30 26 30 27 33 28 51 29 22 30 27 31 28 32 30 33 22 34 42 35 51 36 22 37 33 38 35 39 27 40 25 41 30 42 31 43 31 44 22 45 18 46 25 47 31 48 28 49 45 50 29

Saturation indices

Major formation

Ca

H

H4SiO4

Dolomite

Calcite

3.156 2.920 2.986 2.565 2.845 3.537 2.600 2.818 2.932 3.029 2.894 3.818 2.745 2.637 2.743 3.073 3.073 3.012 2.607 2.917 3.889 3.124 2.688 3.072 2.830 4.103 4.002 3.278 3.082 2.791 2.715 3.463 2.864 3.701 2.516 2.912 3.690 2.799 2.863 2.619 2.991 3.000 2.626 2.437 2.994 2.366 2.996 2.649 2.743 2.360

8.350 8.020 6.810 6.560 7.670 7.790 6.770 7.010 7.010 7.110 7.370 7.000 7.110 7.590 6.780 7.610 7.610 7.410 7.590 8.310 8.340 7.970 7.960 7.940 8.010 8.120 7.890 8.240 8.000 7.800 7.460 8.080 8.200 8.200 8.000 8.070 8.300 7.730 8.000 7.800 7.900 8.150 7.900 8.000 7.680 7.400 8.700 7.800 8.300 7.100

3.181 3.104 2.545 2.699 0.000 0.000 2.619 3.347 0.000 0.000 0.000 0.000 0.000 0.000 2.914 0.000 0.000 3.576 3.548 3.561 3.752 3.554 0.000 0.000 3.151 3.746 3.742 0.000 3.484 0.000 3.347 4.990 3.510 3.710 3.528 0.000 3.329 3.212 0.000 0.000 0.000 3.557 0.000 0.000 3.419 0.000 0.000 0.000 0.000 0.000

1.02 1.17 1.81 1.04 1.38 1.39 1.48 0.41 1.14 1.04 0.54 3.93 0.30 1.38 0.22 0.13 0.13 0.40 1.46 2.18 2.32 0.56 2.50 0.44 1.75 3.24 3.24 0.47 0.66 0.25 1.02 1.18 2.05 1.39 1.96 1.32 1.04 1.28 1.72 0.83 0.52 1.36 1.02 1.06 0.09 1.12 1.96 0.65 2.09 0.55

0.68 0.68 0.75 0.32 0.66 0.68 0.23 0.03 0.43 0.37 0.05 1.93 0.28 0.82 0.01 0.09 0.09 0.01 0.77 1.17 1.14 0.37 1.34 0.40 0.96 1.55 1.58 0.09 0.50 0.26 0.67 0.52 1.13 0.61 1.14 0.79 0.45 0.87 0.78 0.69 0.35 0.68 0.44 0.59 0.12 0.71 1.01 0.34 1.21 0.34

Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Basement complex Basement complex Karoo System Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Karoo system Basement complex Basement complex Basement complex Basement complex Granitic complexes Basement complex Basement complex Basement complex Basement complex Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Basement complex Basement complex Biotite and hornblende Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Biotite and hornblende Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Alluvial, colluvial, residual deposits Basement complex Basement complex Basement complex (continued on next page)

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Table A.1 (continued) Site No.

Log activity Depth (m)

51 28 52 28 53 39 54 31 55 19 56 33 57 41 58 30 59 30 60 28 61 36 62 31 63 22 64 18 Rumphi district sampled sites 65 25 66 30 67 48 68 37 69 30 70 36 71 26 72 44 73 30 74 28 75 27 76 37 77 37 78 30 79 27 80 36 81 22 82 48 83 36 84 30 85 26 86 44 87 37 88 36 89 27 90 32 91 30 92 38 93 22 94 31 95 36 96 37 97 29 98 34 99 20 100 36 101 30 102 31 103 35 104 30 105 61 106 59 107 33

Saturation indices

Major formation

Ca

H

H4SiO4

Dolomite

Calcite

2.924 2.834 2.628 2.834 2.370 3.200 2.712 2.692 2.613 3.131 3.582 2.485 3.111 3.067

7.900 7.900 7.800 7.570 7.400 8.400 7.900 7.800 7.570 8.400 8.000 7.500 7.970 7.900

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 3.603 0.000 0.000 0.000 3.505 0.000

0.61 1.58 1.79 2.07 1.40 1.33 2.00 1.32 0.59 1.86 3.03 1.25 1.40 0.99

0.45 0.74 0.88 1.18 0.81 0.67 1.09 0.73 0.71 0.83 1.48 0.70 0.73 0.47

Basement complex Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Basement complex Basement complex Basement complex Basement complex Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual

2.594 2.646 2.855 2.781 2.991 3.426 3.549 3.707 3.429 3.044 3.584 3.117 3.695 3.136 3.168 3.500 3.782 3.303 3.530 3.051 3.642 3.432 3.240 3.674 3.213 3.288 3.057 3.631 3.225 2.802 3.055 2.677 3.319 3.404 3.354 2.792 3.189 2.894 3.199 3.199 3.500 3.403 3.615

7.520 7.500 8.200 8.000 8.160 8.190 7.610 8.280 7.950 7.740 7.800 7.740 7.950 8.160 7.700 7.540 7.390 7.930 5.960 8.310 7.800 7.555 7.750 7.870 8.120 7.930 7.620 8.380 7.760 8.022 8.170 7.720 7.080 8.270 7.650 6.760 7.090 7.900 6.770 8.230 6.900 6.390 6.730

3.575 0.000 0.000 0.000 3.533 3.643 3.526 3.469 3.506 3.578 3.504 3.460 3.483 3.533 3.578 3.577 3.666 3.554 3.531 3.537 3.552 3.480 3.606 3.742 3.583 3.554 3.502 3.492 3.320 0.000 0.000 0.000 0.000 0.000 0.000 3.548 3.500 3.461 3.500 3.428 3.524 3.417 3.417

0.30 1.16 1.64 1.66 0.75 0.82 1.40 1.19 0.63 0.02 2.67 0.36 1.63 0.46 0.81 2.09 3.05 0.46 0.44 0.71 1.95 0.58 0.09 1.10 0.24 0.64 0.01 0.54 0.16 1.47 1.30 0.50 2.80 1.27 1.76 2.51 1.82 0.93 1.97 0.58 3.13 3.50 3.47

0.11 0.67 0.82 0.78 1.10 0.16 0.73 0.59 0.28 0.22 0.96 0.08 0.78 0.31 0.20 0.91 1.54 0.10 1.05 0.69 0.91 1.11 0.35 2.87 0.19 0.18 0.18 0.28 0.66 1.01 0.76 0.33 1.12 0.16 0.58 1.29 0.70 0.79 0.93 0.43 1.58 1.72 1.75

Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Granitic complexes Mica schists and gneisses Mica schists and gneisses Mica schists and gneisses Granitic complexes Granitic complexes Granitic complexes Granitic complexes Biotite and hornblende Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Basement complexes Basement complexes Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Alluvial, colluvial, residual Basement complexes Basement complexes Basement complexes

development of harmful levels of exchangeable sodium. Even with adequate drainage, special management for salinity control may be used, though plants with good salt tolerance should be selected for C3–S1 groundwaters. On the basis of Wilcox diagram (Fig. 5), groundwater belonged to the excellent to good and good to permissible irrigation water categories. Wilcox (1955) described waters with EC < 750 lS/cm as excellent to good water that might be used for crops under irrigation with little danger of harmful levels of exchangeable Na+. Wilcox described the good to permissible irrigation water as water that might be used to irrigate salt tolerant and semi-tolerant favourable drainage conditions.

deposits deposits deposits deposits deposits deposits

deposits deposits deposits deposits deposits deposits

deposits deposits deposits deposits deposits deposits deposits deposits deposits deposits deposits deposits deposits deposits deposits deposits

deposits deposits deposits deposits deposits deposits deposits deposits deposits deposits

5. Conclusion and recommendation The study has established that groundwater in Karonga and Rumphi districts is neutral to alkaline and mostly freshwater of Ca–HCO 3 type. Some mixed cation-HCO3, (Na + K)–HCO3, mixed cation-Cl, and Ca–(Na + K)–HCO3 water types also existed in some shallow wells and boreholes. Groundwater in both Karonga and Rumphi districts is of low mineralisation which did not show statistically significant variations with respect to depth of shallow wells and boreholes, location and seasonality at 5% significance level. The majority of samples from Karonga District were oversaturated with respect to both calcite and dolomite suggesting meant

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that the majority of Karonga District samples would precipitate (deposit) more of dolomite and calcite. However, a majority of samples from Rumphi District were undersaturated with respect to both calcite and dolomite suggesting that a majority of Rumphi District samples would take more dolomite and calcite into the solution. However, the calculated PCO2 values in soil zones were higher than atmospheric PCO2 suggesting that the groundwater system is open to soil CO2 and that there is possibility of degassing of CO2 during flow, which can increase the pH and subsequently result in the oversaturation of calcite. Groundwater water from both Rumphi and Karonga districts were essentially stable towards calcite and kaolinite stability field suggesting that equilibrium of the groundwater with silicates is an important indicator of the hydrogeochemical processes behind groundwater quality in the study area. Generally, different hydrogeochemical processes like mineral dissolution along with the weathering of silicate and carbonate minerals control the chemistry of the groundwater. The calculated values of SAR, KR and % Na+ indicate good and permissible quality of water for irrigation uses. However, samples with doubtful RSC, unsuitable PI and a high salinity value restrict the suitability of the groundwater for agricultural purposes, and plants with good salt tolerance should be selected for such area. A detailed hydro-geochemical investigation and integrated water management is suggested for sustainable development of the water resources for better plant growth, long-term as well as maintaining human health in the study area. Knowledge of irrigation water quality can also help farmers in choosing suitable alternatives related to potential water quality problems that might reduce production under conditions of use. Appendix A See Table A.1. References Al-Bassam, A.M., Al-Rumikhani, Y.A., 2003. Integrated hydrochemical method of water quality assessment for irrigation in arid areas: application to the Jilh aquifer, SaudiArabia. J. Afr. Earth Sci. 36, 345–356. Allison, J.D., Brown, D.S., Novo-Gradac, K.J., 1991. MINTEQA2, a geochemical assessment model for environmental systems. Report EPA/600/3-91/0-21, USEPA, Athens, Georgia. Alobaidy, A.H.M.J., Abid, H.S., Maulood, B.K., 2010. Application of water quality index for assessment of Dokan Lake ecosystem, Kurdistan Region. Iraq. J. Water Resour. Prot. 2, 792–798. American Public Health Association, 2005. Standard methods for the examinations of waters and waste waters 21st Ed. APHA AWWA-WEF, Washington, DC.. Appelo, C.A.J., Postma, D., 2005. Geochemistry, Groundwater and Pollution, second ed. Amsterdam, Balkema. Ayers, R.S., Westcot, D.W., 1985. Water Quality for Agriculture. FAO Irrigation and Drainage Paper No. 29. Food and Agriculture Organization of the United Nations, Rome, pp. 1–117. Bauder, J.W., 2001. Interpretation of chemical analysis of irrigation water and water considered for land spreading. Personal communication. Montana State University, Bozeman, Montana. Bauder, J.W., Brock, T.A., 2001. Irrigation water quality, soil amendment, and crop effects on sodium leaching. Arid Land Res. Manage. 15, 101–113. Buckman, H.O., Brady, N.C., 1967. The nature and properties of soils. The MacMillan Company, New York, New York. Carter, G.S., Bennett, J.D., 1973. The Geology and mineral resources of Malawi, Bull Geological Survey of Malawi 6, Zomba, Government Printer. Chilton, P.J., Smith-Carington A., 1984. Characteristics of the weathered basement aquifer in Malawi in relation to rural water supplies. In Challenges in Africal Hydrology and water resources, IAHS Publication 144. Domenico, P.A., Schwartz, F.W., 1990. Physical and Chemical Hydrologeology, second ed. Wiley, New York. Doneen, L.D. 1964. Notes on water quality in agriculture. Published as a water science and engineering paper 4001, Department of Water Science and Engineering, University of California. Drever, J.I., 1982. The Geochemistry of Natural Waters. Prentice-Hall, Inc., Englewood Cliffs, NJ. Eaton, F.M., 1950. Significance of carbonate in Irrigation waters. Soil Sci. 67 (3), 128–133.

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