OR/15/009 Risks to groundwater supplies: a source-pathway-receptor framework for Sierra Leone

Lapworth D J, Carter R C, Pedley S and MacDonald A M. 2015. Threats to groundwater supplies from contamination in Sierra Leone, with special reference to Ebola care facilities. British Geological Survey Internal Report, OR/15/009.

To help identify the risks of pathogen contamination in groundwater supplies used for drinking it is helpful to frame the problem within a source, pathway receptor framework. This provides the basic framework for most groundwater risk assessments. The causes of groundwater quality degradation may be separated into those related to the source of the contaminants and those which govern their transport i.e. the pathways, into and through the water environment. The receptors in this particular study are taken to be the water supplies used for drinking – mainly groundwater sources, (such as wells boreholes and springs) but also small streams and swamp areas since they are also used in Sierra Leone. Key potential sources, pathways for groundwater receptors are summarised in Table 10.

For microorganisms in faecal and other waste materials, the main barrier to their movement into groundwater is the soil and unsaturated zone. As discussed earlier, once in the subsurface, a complex interaction of other physical, chemical and biological factors control the survival and mobility of the microorganisms (Pedley et al., 2006). Once the microorganisms has reached the groundwater the main factors that enable attenuation are dilution and the groundwater travel time to the various water supplies, which as described above can be rapid in Sierra Leone.

Groundwater vulnerability

In recognition of the importance of protecting groundwater resources from contamination, techniques have been developed for predicting which areas are more likely than others to become contaminated as a result of human activities at the land surface. Once identified, areas prone to contamination can be subjected to certain use restrictions or targeted for greater attention. Groundwater vulnerability is a term that has been in use for more than 40 years. A general definition is given by the US Natural Research Council (1993): “Groundwater vulnerability: the tendency and likelihood for [contaminants] to reach [a specified position in the groundwater system] after introduction at [some location].” Most other definitions replace the phrases in brackets with specific terms. The most commonly used definition (e.g. U.K and most other European countries) is: “The tendency and likelihood for general contaminants to reach the water- table after introduction at the ground surface”

The vulnerability of groundwater to pollution depends upon:

• The time of travel of infiltrating water
• The contaminant attenuation capacity of the soil and geological materials through which the water and contaminants travel

If the contaminant source is at the ground surface and the source is the water-table then the main pathways to consider are the soil and the unsaturated zone. Since the soil is biologically active many pollutants can be attenuated. However, if the contaminant source is buried beneath the soil, then only the unsaturated zone should be considered where there may be less opportunities for attenuation. In general, fractured aquifers with shallow water tables are assessed to be extremely vulnerable (e.g. O Dochartigh et al. 2005).

If groundwater protection strategies are considering the water supply sources as the receptor rather than the groundwater, then the travel times and attenuation potential of the saturated aquifer should also be taken into account. In areas where fracture flow and rapid transit dominates (such as in shallow tropical soils with heavy rainfall), the travel time horizontally through the shallow sub- surface can be very short, this pathway may not have significant attenuation potential.

Table 10 Hazard sources and pathways for contamination of water points in Sierra Leone (adapted from Lapworth et al. 2015a)

Component	Category	Risk factors
Regional considerations	Population density Land use category Physical relief Rainfall amount and intensity
Sources	Municipal/household including domestic livestock	Surface sources: Open defecation from humans and animals Surface waste sites Sub-surface sources: Latrines Septic tanks Soak-aways Waste pits Cemetery or other burial sites (Open) sewers Other potential hazard sources: Market places, Abattoir waste, both liquid and solid
	Hospital or Treatment centre	Liquid waste discharge to soak-aways/surface channels Solid medical waste disposal Latrines/septic tanks on site
	Industry e.g. mining	Process plant effluent Solid waste disposal Storage tanks Site runoff
Pathways	Horizontal and vertical pathways in unsaturated and saturated zone	Shallow sub horizontal pathways in tropical soil''': Tropical soils, e.g. Plithosol/Ferrasol horizons present Shallow depth to water table Thin soils and low organic matter content Natural rapid bypass from tree roots and burrows Vertical and horizontal pathways in saturated zone: Thickness and maturity of weathered basement zone Fracture size, length and density in the more competent bedrock below weathered basement
	Local/ headwork pathways	Lack of dugwell headwall and/or lining Lack of well cover Use of bucket and rope – soil/animal/human contact Gap between apron and well lining Damaged well apron Propensity for surface flooding Gap between borehole riser/apron Damaged borehole apron Eroded or de-vegetated spring backfill

Extreme vulnerabilities are associated with highly fractured aquifers which offer little chance for contaminant attenuation. The likely vulnerabilities of a range of broad categories of aquifer types relevant to Sierra Leone are shown in Table 11.

Table 11 Generalised pollution vulnerability for hydrogeological environments found in Sierra Leone (adapted from Lawrence et al., 2001).

Hydrogeological environment		Travel time to saturated zoneb	Attenuation potential	Pollution vulnerability
Weathered basement	Permeable tropical soilsa Thick weathered layer (>20m) Thin weathered layer (<20m)	Days-weeks Months-years Weeks-months	Low-High High Low-High	High-Extreme Low High
Thick sediments associated with rivers and coastal regions	Shallow layers Deep layers	Weeks-months Years-decades	Low-high High	High Low
Minor sediments associated with rivers	Shallow layers Deep layers	Days-weeks Months-years	Low-high High	Extreme Low

^ae.g. Ferrasol or Plinthosol horizons present, ^bhigher travel times may operate for short periods of time during high intensity rainfall and when water tables are high, equally longer travel time are also possible in some settings

Conceptual models of pathways for groundwater contamination

This section focuses on summarising the main types of drinking water sources or ‘receptors’ used in Sierra Leone, the key sources of hazards, both surface and subsurface sources, and the major pathways for transmission of pollutants to groundwater receptors. These are summarised briefly in Table A2 (see appendix) and through the use of simplified schematic diagrams of key processes and accompanying text in the following section. These conceptual models show worst-case scenarios under high water table conditions in basement terrains, i.e. typical conditions found in August-September. Surface water sources are of generally poor quality, and are highly vulnerable to surface sources of contamination. Apart from the public supply to Freetown which is piped from the Guma Valley reservoir, most domestic water used in Sierra Leone are from hand dug wells, boreholes make up less than 10% of groundwater sources. The supply and treatment from public supplies are intermittent at best, and household treatment is essential in Freetown and elsewhere. Household treatment of groundwater sources is also highly intermittent and is only likely to be more widespread during outbreaks of water-borne disease.

Figure 23 shows a schematic of the main drinking water sources for Sierra Leone which include wells, boreholes and surface water. Springs are also used in some locations, for the purpose of this report these can be considered analogous to unlined traditional wells as they essentially access the same shallow groundwater zone and are highly vulnerable to open defecation.

Figure 23 Groundwater receptors and key groundwater zones typically found in Sierra Leone and elsewhere in tropical basement terrains. Sources of contamination and key pathways have been greyed out for clarity. High groundwater level conditions with highest risks are presented.

As well as the three key sources of drinking water (labelled 1-3 in Figure 23) two key groundwater zones have also been highlighted: i) the ‘shallow groundwater zone’ (typically less than 20 metres below ground level - mbgl), which is typically accessed by wells and springs and has shorter residence times and is susceptible to rapid pathways in tropical soils; ii) the ‘deeper groundwater zone’ which is accessed by boreholes, and in a few cases by deep wells, and has much longer average residence times, typically 20-30 years (Lapworth et el., 2013), and is greater than 20 mbgl within the higher storage weathered basement and fractured basement. A low permeability zone is located above the higher storage weathered basement zone which is low yielding and is therefore not suitable for groundwater abstraction. Surface water sources include streams, swamps and in the case of Freetown a purpose built reservoir.

The key sources of hazards relevant to groundwater and surface water supplies are highlighted in red in Figure 24. These include surface sources of contamination, open defecation by humans and livestock (1), solid waste (2), soil microbes (3) - some of which are opportunistic pathogens or more prevalent in the environment such as V Cholera, liquid waste from domestic and municipal sites applied to the surface (4). These sources, in most cases, will be largely attenuated in the biologically active soil zone through biological and physio-chemical processes and are therefore are conventionally viewed as less of a threat to groundwater quality. However, where these sources are widespread and essentially diffuse (such as the case in urban settings) and the climate is very wet, such as in Sierra Leone, these should be considered a significant source of hazard to groundwater and surface water supplies. This is a particularly important hazard source for wells where ropes and buckets are used which come in to regular contact with surface sources of hazards.

Subsurface sources include cemeteries (5), pit latrines (6) and open sewers and drains, and in the context of Ebola, burial and waste disposal pits (7). These sources, by their very nature, do not benefit from potential hazard attenuation in the soil zone and are closer to the groundwater table, and highly permeable tropical soil zone, and therefore pose a considerable risk to water sources. The sanitation coverage of Sierra Leone is low, and therefore overall the risks from pit latrines may be less compared to other countries in SSA.

Figure 24 Schematic showing key sources of hazards relevant to groundwater and surface water supplies. Receptors and pathways have been greyed out for clarity. High groundwater level conditions with highest risks are presented.

Surface water sources are particularly affected by surface sources of contamination (1-4) as well as subsurface sources 6 and 7, see Figure 24. Pit latrines and open sewers may drain directly into surface water courses, and the contents from pit latrines are sometimes disposed of directly into surface water bodies, giving rise to considerable risks to downstream users. The current pit emptying practices, if/when they happen, are not well documented for Sierra Leone.

Figure 25 summarises the key pathways, highlighted in orange, including surface and subsurface pathways for migration of pollutants from sources to receptors. Surface pathways include surface runoff (1) which can contaminate surface waters and poorly constructed wells, bypass pathways for contamination of well and spring collectors by ropes, buckets used to draw water (2). Shallow sub-surface pathways include vertical soil flow from surface (3) and subsurface sources (4) where there is hydraulic continuity, e.g. from a liquid discharge or from a buried source such as a pit latrine, cemetery or buried waste.

Figure 25 Schematic highlighting key pathways for hazard migration to groundwater sources. Pathways and receptors have been greyed out for clarity. High groundwater level conditions with highest risks are presented.

Very rapid horizontal pathways exist in the shallow tropical soil zone (5), which may be laterally extensive, providing transmisivities in excess of 300 m²/day. Rapid vertical pathways also exist due to the presence of natural macro-pores e.g. from burrows and tree roots (6), which can reach significant depths in places. Combined, these more rapid pathways make shallow wells and spring sources particularly vulnerable to contamination and are increased during high water table conditions or when soil infiltration capacity is exceeded. Horizontal saturated groundwater flow, both in the lower permeability horizon above the weathered basement (7) and in the weather basement and fractured basement (8) is a pathway which can affect deeper groundwater sources such as boreholes. These pathways are slower and longer and provide the greatest attenuation potential for hazards.

In areas with red tropical soils groundwater flow exhibits extremely high permeability characteristics, i.e. very rapid transient pathways may operate for short periods of time and show sudden changes in permeability. The combination of high rainfall and the prevalence of these types of tropical soils suggest that a significant part of Sierra Leone, and neighbouring regions, may be susceptible to these types of extreme hydraulic flow conditions. This, combined with the fact that diffuse open defecation is widespread, cast doubt on the simplistic use of single minimum separation distances from particular hazard sources, and requires further investigation.

Key source-pathway-receptor considerations for water points in Sierra Leone: Given the low sanitation coverage in Sierra Leone, surface sources of faecal contamination are likely to be as important as pit latrines and other buried sources. Very rapid shallow lateral pathways in the tropical soil zone under high water tables and intense rainfall conditions, with limited potential for attenuation, are a major pathway for contaminant migration to shallow water points. Pathways which bypass natural attenuation, either from buried sources or due to the use of ropes and buckets, are a particular risk for traditional and improved wells. Surface waters and shallow groundwater receptors are most at risk from hazard sources; water points such as boreholes which access deeper groundwater have greatly reduced risk of contamination.