Abstract

A quality decline trend is suspected to be ensuing in the water originating from boreholes in Langata sub-County; a region located to the south of Nairobi, the capital city of Kenya in East Africa. Despite the existence of this worrisome suspicion, no investigation has been conducted to assuage the fears of the exposed population. This situation however contradicts the great augmenting role of borehole water delivered by vendors to consumers as a coping strategy against the pervasive drought induced water shortage challenge afflicting households in Langata. Hence, a question arose as to whether the identified past drought events have had any chance of influencing the ongoing declining trend of the area’s ground water quality. The purpose of this study was therefore to assess the impact of historical drought events on the ground water quality in Langata Sub County. The profile of drought indices was superimposed over the area’s time series geochemical water quality indices’ profile. Further, the computed area’s groundwater potability grade was used to estimate the probability of water quality deterioration due to drought impact, returning a value of 43.65%. It was found that indeed, ground water quality in the area is on a declining mode. Since water is an elixir of life; the finding from this study is expected to trigger an establishment of a water quality surveillance initiative as a safeguard to public health.

1. Introduction

Water is an elixir of all life forms on Earth ¹. Now-a-days; groundwater is a significant source of water for human consumption ². It is supplying nearly half of all drinking water in the world ³. Scholars like those in ⁴ have argued that, groundwater is a reliable source of fresh water for mankind’s water needs. Its slow response to meteorological conditions makes it a big buffer against climate variability including drought according ^{5, 6}. It is estimated that 35% of the earth’s surface does experience some form of drought in any given year ⁶. This puts into doubt whether groundwater will meet the increasing water demand ⁷ given the emerging depletion threats from a reduced recharge rate in the face of an increasing demand ⁸.

Already, in the African continent, almost 44% of the people lacks access to safe water due to the region’s susceptibility to rainfall variability ⁹. Exacerbating this lack of safe water access is the threat from climate change which is suspected to be fueling groundwater quality decline ^{10, 11, 12, 13}. Dietary exposure of contaminants through drinking water is a potential source of nearly 75% of the world’s waterborne diseases ¹⁴. In an effort to put in measures to protect groundwater quality; a lot of research work has gone in to investigate the sources of groundwater contaminants with an aim to ensuring the adherence to the World Health Organization’s safe drinking water guidelines ^{15, 16, 17}.

In the mid 1980s, worker in ¹⁷ pointed out that pesticide was a potential contaminant of ground water, a claim later affirmed by ¹⁹ who added that insecticide residues were also found in soil and fruits thereby affecting farmers’ health. In the early 1990s, scholar in ²⁰ added their voice, this time blaming pesticides infiltration into groundwater thereby causing adverse health effects on the exposed people. Weighing in on this point, workers in ²¹ further emphasized the danger people faced from exposure to pesticides in drinking water. Following this, the American Conference of Governmental Industrial Hygienists, a group of industrial hygienists and other occupational health safety professionals dedicated to promoting health and safety, on their part, recommended threshold limit values for chemical substances and physical agents ²². That is why the workers in ²³ recently recommended for the application of nanofiltration for removal of pesticides, nitrate and hardness from ground water to meet drinking water standards as per guidelines in ²⁴. Further, a decade ago, scholars in ²⁵ noted that, the deterioration of water quality within the supply chain infrastructure including corrosion of pipes was on the rise. According to workers in ²⁶ the galvanized steel pipes are plated with zinc, which usually has 1% of Cadnium, an element found also in the soldering of pipe-fittings. For this reason, an appropriate storage coupled with sustained quality surveillance is crucially important ²⁷.

Other scholars like those in ²⁸ proposed for the removal of pharmaceuticals during drinking water treatment processes using advanced technologies such as ozonation, granular activated carbon and membrane filtration techniques. This is in line with the toxicological evaluation of hazard, dose-response, exposure assessment for risk characterization for tolerable daily intake of chemicals ¹⁶. Already, the World Health Organization has isolated more than 134 chemical substances on its watch list which every country’s water standards must abide to maintain their safe limits of acceptable daily intake ¹⁵. The forcing from the combination of anthropogenic global warming and urban heat island effects in cities has equally been associated with the catalysis of groundwater quality decline through biogeological reactions in the observation by ²⁹. In addition, the over pumping of subsurface aquifers also has been found to be contributing to groundwater quality distortion according to workers in ³⁰. This assertion was affirmed in the 1999-2004 study on water-quality assessment of the High Plains aquifer, the America’s Ogallala aquifer that underlies eight different states, stretching across America’s High Plains from South Dakota down to Northern Texas ³¹. In the coastal cities, the study by scholars in ³² established that groundwater is very susceptibility to degradation due to salination which requires that it must be subjected to desalination procedures before use.

The same desalination procedures were also recommended for the removal of radioactive contaminants in groundwater like Uranium by workers in ^{33, 34, 35} in line with the advice from International Agency for the Research on Cancer ³⁶. The World Health Organization’s guidelines on Uranium sets the safe concentration level for human intake as 15 µg/L. Several studies on the effect of Uranium on human health have been done which have revealed its disease causing nature. Excess Uranium intake is associated with leukemia ³⁷, stomach cancer ³⁸, urinary cancer ³⁹, cancer of urinary organs ⁴⁰, kidney toxicity ³³ and bone toxicity ³⁴. In fact, the presence of Uranium in groundwater is of a significant concern in a number of countries including; USA, Canada, Germany, Norway, Greece and Finland where its high chemical toxicity and lethal effects on human skeleton and kidney have been identified ^{33, 34, 41}.

In the oil and gas industry, scholars have also identified shale gas development as posing a potential negative impact on groundwater quality ⁴². The fracking technology impact on groundwater quality was first raised by ⁴³ who documented a felt seismicity associated with the shale gas hydraulic fracturing in Europe. Just recently, workers in ⁴⁴ affirmed the same concern through the finding from the study on the baseline groundwater monitoring for shale gas extraction in Wysin, Northern Poland. The study established that there is a potential contaminant flow into groundwater aquifers from fracking activities, posing a real danger to groundwater users. In developing countries, pit latrines have been found to pose a real challenge to groundwater quality according to study results by workers in ^{45, 46}. From the foregoing, it can be deduced that groundwater quality is under threat from multiple sources including from climate change impacts. Seemingly, it appears that to date, little has been documented on the measurable influence of drought episodes on groundwater quality. Before attempting to address this gap, this study first presents the background information on the need to control groundwater quality.

2. Groundwater Quality Control

Scientific records show that groundwater is the largest distributed store of freshwater but one which continues to expose consumers to varying doses of heavy metal toxicity ⁴⁷. Notwithstanding the toxicity challenge, it fascinately plays a critical role in sustaining and enabling human adaptation to climate change ². One of the contributing factors to its quality problem is the continuing subsurface thermal regime changes attributable to the global warming. Unfortunately, this fact is yet to be fully understood by researchers as asserted by ⁴⁸. Consequently therefore, a region like East Africa with high climate variability records could be exposing her people to groundwater quality danger ⁴⁹. The danger notwithstanding, groundwater is a credible source of emergency freshwater everywhere ⁵⁰.

Because of this, surveillance on groundwater quality has become a mandatory undertaking ⁵¹ particularly in Africa where reliance on groundwater is growing ⁵² even though the full knowledge on its quality regime still remains a challenge ⁵³, a point recently affirmed by ⁵⁴ for the Kenya’s case. Actually, groundwater quality management is becoming a complex matter for three reasons; one, the transient nature of standards of drinking water, two, the increasing pollution threats and lastly, the rising water shortage challenges in communities. It is these complexities that drinking water treatment processes are always on regular reviews to help remove pesticides, hardness, nitrates and natural organic matter. In addition, a continuous monitoring and surveillance measures are also always put in place to track water quality to ward off accidental ingress of contaminants after water is treated ⁵⁵. For the groundwater, the contaminants originate from geological conditions, industrial activities, and agricultural processes.

The contaminants contain; microorganisms, inorganic matter, organic matter and radionocludes. It is sometimes insinuated that, the inorganic matter holds a greater portion in the contamination problem compared to the organic chemicals according to ²⁴. Some inorganic chemicals are in mineral form of heavy metals. When heavy metals accumulate in human organs and nervous systems they cause impairment in their normal functions. The culprit heavy metals include; Lead (Pb), Arsenic (As), Magnesium (Mg), Nickel (Ni), Copper (Cu) and Zinc (Zn) all of which are continuing to pose serious health issues on mankind if ingested beyond the tolerable daily limit. Their presence is measured using approved methods ⁵⁶ by applying Flame Atomic Absorption Spectrometry (FAAS) as discussed in ⁵⁷.

The standards include; Manganese ⁵⁸, Copper ⁵⁹, Iron ⁶⁰, Zinc ⁶¹, Cadnium ⁶², Chromium ⁶³, Lead ⁶⁴, Arsenic ⁶⁵ and Mercury ⁶⁶. These heavy metals have varying implications on human health when ingested beyond daily tolerable limit. For example, Cadnium (Cd) and Chromium (Cr) over exposure is implicated with the cardiovascular diseases, kidney related problems, neurocognitive diseases and cancer according to some past epidemiological findings in Peninsular Malaysia by workers in ⁶⁷. Further, Cadnium (Cd) which ordinarily occurs naturally in rocks and soils is known to enter water when there is contact with soft groundwater. It can also be introduced by paints, pigments, plastic stabilizers, mining and smelting operations and industrial operations such as electroplating and fossil fuels, fertilizers and sewerage sludge disposal ²⁶.

On the other hand, Lead (Pb) is associated with causing delay in the physical and mental growth in infants. Mercury (Hg) intake has been attributed to poisoning with skin pathology, cancer, damage to both kidney and liver ⁶⁸. Additionally, Mercury compounds are classified by International Agency for Research on Cancer (IARC) as being in group 3 carcinogens ^{36, 69}. In addition, poisoning by Arsenic (As) found in groundwater, fish and sea foods is a global problem ⁷⁰. For example, in Bangladesh, chronic Arsenic poisoning has become a major public health problem where its concentration in water is above the WHO provisional guideline value for drinking water of 10 mg/L ⁷¹. Chronic Arsenic exposure may lead to irreversible damage to several vital organs; moreover Arsenic is established as carcinogen ⁷². Cessation of exposure to Arsenic and providing Arsenic safe water are currently the mainstays of management of arsenicosis patients ⁷³.

The purpose of analysis methods applied to water resources is to identify potential problems before they give rise to serious adverse health effects ¹⁷. These methods include the analysis of different parameters such as pH, turbidity, conductivity, total suspended solids (TSS), total dissolved solids (TDS), total organic carbon (TOC), and heavy metals ^{56, 57}. These parameters can affect the drinking water quality, if their values are in higher concentrations than the safe limits set by the World Health Organization (WHO) and other in-country regulatory frameworks ²⁴. Therefore, the investigation of the drinking water quality by researchers and governmental departments has been performed regularly throughout the world ^{74, 75, 76}. The big question is, why do all these tests? The tests are done because drinking water should have a composition favorable to human health and be devoid of harmful substances especially the control of the level of concentration of the important basic inorganic elements that include ;Calcium, Magnesium, Sodium, Potassium and Bicarbonate ions.

These inorganic elements are considered as being the main components in a typical natural freshwater as they form over 90% of the dissolved substances. Freshwater quality and the content of inorganic components depend on the type of substratum. The quality of groundwater may pose a threat to consumers’ health, mainly because of high nitrate concentrations ⁷⁷. Nitrates may cause methaemoglobinemia in infants and little children and diseases of alimentary tract and hypertension in adults ⁷⁸. Long exposure to high nitrate concentrations may be carcinogenic ⁷⁹. Drinking water is an important source of Calcium and Magnesium in diet ⁸⁰. Both Calcium and Magnesium participate in many physiological processes in human organism at subcellular, cellular and tissue level ⁸¹.

Their deficit leads to hypocalcemia and hypomagnesemia ⁷⁸. Chronic hypocalcemia may result in osteoporosis ⁸². Moreover, hypocalcemia increases the risk of cerebral stroke and leads to an increase of blood pressure ⁸³. Magnesium deficit in human body contributes to diseases of blood circulation system, disturbs heartbeat rhythm, causes vertigo and muscle spasms. High Magnesium intake may reduce the occurrence of colorectal cancer in women ⁸⁰. Systematic uptake of higher amounts of Phosphates in drinking water may result in unfavorable effects on bone metabolism and disturbed Calcium-Phosphate equilibrium ⁸⁴. But generally; there is need for risk awareness and communication about heavy metal contamination in groundwater ⁸⁵.

In the physical tests, turbidity measurement is listed first. It indicates the cloudiness of water due to the presence of particles. Remotely, it is a proxy indicator of the content of disease causing organisms in water which may originate from soil runoff. The standard maximum level by WHO is 5 Nephelometric Turbidity Units (NTU). Another important physical feature of water is the electrical conductivity or the specific conductance measuring the water’s ionic content. Electrical conductivity is simply the ability of water to convey an electric current. The presences of dissolved solids (the inorganic) like; Calcium, Chloride and Magnesium in a water sample carry the electric current through the water. The maximum allowable level of conductivity by WHO is 1000 μS/cm (1000 microsiemens/centimeter). Conductivity does not have a direct impact on the human health but the measured value can be used to; estimate the existence of minerals such as Potassium, Calcium and Sodium and estimate the amount of chemical reagents to be used in water treatment.

It is said that high conductivity may lower the aesthetic value of water by raising the level of mineral taste. In the agricultural sector as well as in the industrial set ups; high conductivity may cause corrosion of equipment such as boilers. This may also affect home appliances such as water heater systems and faucets. Another important physical water parameter is the P^H .Measurement of water P^H is an indicator of acidity or alkalinity. Water sample is considered as acidic if the P^H value is below 7.0 and when above this, then the water sample is alkalinic. Acidic water can lead to corrosion of metal pipes and plumbing systems. On the other hand, alkaline water is an indicator of presence of disinfectants. The drinking water should have a P^H value ranging between 6.5 to 8.5 as per World Health Organization’s guidelines. In Africa, studies of metal pollution are scarce ⁸⁶, yet there are growing evidences that problems of heavy metals are posing increasing risks to the residents in the continent ⁸⁷. In South Africa, a study by workers in ⁸⁸ on groundwater toxicity established its unsuitability for drinking purposes.

In Kenya, workers in ⁸⁹ reported on heavy metal toxicity exposure among the inhabitants around the Kenyan side of Lake Victoria shoreline, which was a parallel finding as that done recently by scholars in ⁹⁰ for the Uganda side. But the biggest contamination challenge for Lake Victoria water is from Nitrogen inflow from the upstream environments. The Nitrogen enrichment of Lake Victoria stems mainly from sewage effluents and river discharges carrying residual farm inputs according to workers in ⁹¹. This has caused groundwater Nitrate contamination in the larger Kisumu city and its environs like Kano Plains ⁹² which fortifies the earlier findings by ⁹³ on heavy metals toxicity problem. Even in Kakamega County, a northern neighboring city to Kisumu has the same problem with its groundwater quality as per a study by scholars in ⁹⁴. In Kiambu County of central Kenya region, workers in ⁹⁵ reported a seasonal variation in physicochemical and microbiological groundwater quality which was similarly reported by ⁹⁶ for Tharaka Nithi County in the Eastern Kenya region. In 2016, a study was conducted in Ongata Rongai town in Kajiado County which is located to the south of Nairobi City County. It observed that groundwater quality is largely affected by the proximity to sanitation facilities ⁹⁷ which is similar to the findings from the Republic of Malawi as was reported by workers in ⁴⁵.

In Nairobi City County itself, workers in ⁹⁸ while using spectroscopic technique, established that Nairobi River System’s waters are polluted with a matrix of toxic metals. It should be noted that it is the same river system that plays the groundwater recharging role thereby indicating the proximal potential the toxicity challenge may confer to the receiving aquifer. In 2007, a study by ⁹⁹ had also reported heavy metal pollution around Dandora Municipal solid waste dump site which is located to the north of Nairobi’s city centre. This again explains the finding by ¹⁰⁰ which reported that African leafy vegetables grown in the urban and peri-urban zones of Nairobi city have high presence of heavy metals. On the ground water context, a study by ¹⁰¹ using laboratory testing method had established that Langata sub County region’s groundwater was too alkaline having recorded the highest P^H value of 8.75 relative to the other regions of Nairobi.

This added to the earlier finding using same method by ¹⁰² which had flagged the issue of Flouride over-concentration in Nairobi city’s groundwater. From the foregoing, it is noticeable that the augmenting role of groundwater resource is plagued by contaminants’ exposure problem inevitably calling for the establishment of a quality control early warning initiative framework. Accordingly and as a first step, this study aims to develop an understanding of the influence of drought events action on groundwater quality decline within Langata sub County. This will be reinforced by the calculation of the probability of the effect on areas’ groundwater quality grade by drought events. This proposed method has an advantage over the laboratory based approaches used in the previous studies because it is simply a desktop based activity. The rest of the remaining sections of this paper are; Section 3 presents materials and methods, Section 4 outlines the results and discussions while section 5 is the conclusion.

3. Materials and Method

Langata sub County is found in the south of Nairobi city center, located approximately at 1o 22’0” S, 36o44’ 0” E. Its topography height range is between 1,600m to 1,850 m above mean sea level. It covers an area of about 196.8 km² in area, with a tolerable temperate tropical climate throughout the year see location map in Figure 1 below. In terms of population growth, the record shows that in a span of 50 years, the population of Nairobi city grew 12- fold, from around 293,000 inhabitants in 1960 to about 3.4 million in 2010 as reported by ^{103, 104}. Between 1948 and 1999 workers in ¹⁰⁵ also reported that the city population grew by 12.2%.

Equally, in terms of urbanization, the city was 3.84 square kilometers in 1910 compared to its current area of 696 square kilometers according to ¹⁰⁶ as cited by ¹⁰⁷. For the study area, record in ¹⁰⁸ shows that Langata sub County had 174,314 or 5.7% of the 3,078,180 city’s residents spread in 52,656 households. This rapid population growth and a steep urbanization rate for Nairobi city is described by ¹⁰⁹ as being typical of a sub-Saharan Africa (SSA) urban centre characteristic. Geologically, Nairobi city area lies immediately east of the Kenyan rift valley. As such, volcanic, mainly lava flows dominate the geology of this area. These are of the Cenozoic age, overlying a basement of folded schists and gneisses of the Mozambique belt ¹¹⁰.

In terms of water supply, Nairobi City County imports 80% of its bulk water supply from Ndakaini and Sasumua dams located more than 50km away, see Figure 1. There are a number of challenges on the water supply system. Firstly, the current bulk system is not reliable due to drought impacts and siltation of reservoirs arising from deforestation in the upstream catchments. Further, there is a considerable inefficiency in the distribution system causing nearly 50% unaccounted water associated mostly with pipe leakages and illegal connections. As a result, the city is under severe water rationing ¹¹¹. To cope, residents have turned to groundwater being supplied by both publicly and privately operated boreholes. On a larger scale, the government has also responded by drawing a robust long term water master plan for the city which is hinged on the Integrated Urban Water Management (IUWM) system up to year 2035 time horizon ¹¹². These ambitious plans are faced with additional challenges like; climate variability and uncertainty in the projected water supply and demands. This means, sources like the informal water market must also be included in the water plan. But their sources of water must be within the safety guidelines.

Records show that Nairobi boreholes abstract their water from unconfined, confined and perched aquifers with variable chemical quality. This means the water has varying geochemistry due to localized geochemical processes and possible faulting compartmentalization. This raises questions on the quality of ground water for human consumption. A number of groundwater quality investigation studies have been done in different parts of the city most of which have established a high fluoride concentration beyond WHO recommended guidelines which proposes r mixing of groundwater with surface water in a ration of 1:1 to improve its potability. With the increasing water demand, the demand for groundwater is increasing in Nairobi. This demand must be met with a good safety plan in the form of water quality monitoring and control according to ¹⁰².

For drought risk quantification, the monthly rainfall data for the period 1957 to 2013 (57 years) was sourced from Wilson Airport; Kenya Meteorological Service station number: 9136130. This was backed up with the discussion on 2016 drought by ¹¹³ that extended the data period from 2013 to 2016.Additionally, on groundwater quality index computation and grading, the borehole drilling data was sourced from Water Resources Authority (WaRA) offices in Nairobi’s industrial area. The borehole commissioning data for Langata sub County region for years 1982 to 2017 was issued which had a total of 137 borehole files. On analysis using the perspective of hydrogeochemical parameters, 98 files missed on one or more of the eight inorganic parameters; Potassium K⁺, Sodium Na⁺, Calcium Na⁺, Iron Fe²⁺,Flouride F^-, Chloride Cl^-, Sulphite SO₄^-2 and Electrical Conductivity Ec (µS/cm).This left 39 sample files for analysis.

The Standard Precipitation Index formula usually expressed as; was used to characterize drought in Langata. A detailed procedure is explained in the paper by ¹¹⁴. In this formula is the annual rainfall of the i-th year and is the mean annual rainfall over the full period, while ɗ is the standard deviation. The SPI values range between -2.5 and 2.5 with 0 being the turning point between wetness and dryness. This study is interested only on dryness and so any SPI value below 0 is counted as a dry year. On groundwater potability estimation, the study deployed the guide by ¹¹⁵. The full procedure of how the above 8 selected inorganic chemical parameters from 39 boreholes making 312 samples for analysis is explained in the paper by ¹¹⁶ From the results of groundwater potability evaluation and drought characterization Table 1 below was prepared. Using the approach explained in the handbook by ¹¹⁷ in pages 133 and the Z-Normal Distribution Table as presented in pages 147-148, the water quality index of the 39 boreholes used in the quality analysis were taken as the variables to be analyzed.

The evaluated overall groundwater quality for Langata according to workers in ¹¹⁶ is grade “C” which falls in the grading band 40-60 with an actual score of 53.18.It should be noted that grade “A” band is 0-20; with grade “B” falling in the band 20-40 and any score beyond 100 is unsuitable for drinking. In grade “C”, the upper limit of the score is 60.Therefore, to calculate the probability of exceedance of grade “C” whose maximum value score is 60, the following pieces of information are worked out in MS Excel as presented in Table 1 here: the mean of the dry spell borehole water quality index and the standard deviation. For a normally distributed data, the probability of exceeding a standard value set is;

Where is computed from It is the which is read out from Z-Table to give the probability of exceedance in percentage.

4. Results and Discussions

From the probability of exceedance formula as above presented the, is 60 and reading from Table 1 above for the dry year mean, is 55.13 and the standard deviation σ is 29.92. Therefore, is 0.16 as read out from Z-Table giving the probability of exceedance in this case as 0.4365 from the Z-Normal Distribution Table. This probability value therefore when expressed as a percentage by multiplying by 100 means that the drought events in Langata have up to 43.65% chance of lowering the ground water quality by exceeding the grade “C” limit of 60.Using values in Table 1 above, the profile of water quality and drought index values were plotted in Excel as presented in Figure 2 below. The blue line is the water quality profile line for the data period and the red line is the rainfall performance profile from the SPI values in Table 1. For the rainfall, values below zero SPI value are drought events. It can be deduced from Figure 2 that as the drought frequency increases so does the ground water quality continues to deteriorate showing a quality decline mode.

5. Conclusion and Recommendation

The objective of this study was to establish if the drought events in Langata have had an effect on the deterioration of groundwater quality. In addition, it sought to estimate the probability of the effect on quality. Findings from the study reveal that, the past historical drought events have had a significant role in the decline of the groundwater quality based on the assessment of geochemistry parameters. In 1983, the borehole file with serial number (S/N 465) in Table 1 was having grade potability. But regrettably, in 2005, the groundwater quality in Langata reduced drastically with the 2016’s record being the worst ever at grade F (163.38), see Table 1 above for borehole with the serial number (S/N 1383).Consequently with respect to the water from this particular borehole (S/N 1383), it becomes unsuitable for drinking purposes.However, this study suspects that the consumers may not be aware this anomaly. In terms of the probability of drought events’ influence on the groundwater quality decline, the computation returned a value of 43.65%. This means that, the groundwater quality in the area is substantially susceptible to decline with every drought event. From the findings therefore, it is hereby recommends that, a groundwater quality control and monitoring scheme be started in Langata sub County to safeguard public life from exposure to any form of toxicity.

Acknowledgements

The study team acknowledges with a lot of gratitude the moral support that was extended by the management of the Institute for Climate Change & Adaptation at the University of Nairobi during the period of research. However, this study was not sponsored. Therefore, none of the authors has a direct interest whatsoever.

References