X.3 Soil and Groundwater Assessment
A reliable knowledge of existing soil and groundwater conditions is important in sanitation planning and a key factor in the selection of appropriate technologies, especially where infiltration-based sanitation systems such as Single Pit Latrines S.3 or Soak Pits D.10 are to be used. Soils with a high infiltration capacity can be desirable from a technology perspective, but may be undesirable from a health and safety perspective, as they increase the risk of groundwater contamination. On the other hand, more compact, impermeable soils such as clay may severely limit infiltration and making drainage almost impossible. This has a direct impact on the filling rate of pits and the quality of faecal sludge. The main danger is the contamination of groundwater used for drinking water by pathogens of faecal origin. When pit latrines are densely concentrated in an area and shallow aquifers are used as a source of drinking water, nitrate (which should not exceed 50 mg/L in drinking water according to World Health Organization guidelines) may also be a health hazard.
When a settlement or camp is built, and too many trees are felled, soil can lose permeability through compaction, resulting in an increased runoff and a higher risk of flooding. Infiltration can also be reduced, which results in less recharge of shallow aquifers. At the same time, the installation of sanitation infrastructure increases the risk of surface and groundwater contamination. Two flows of possible bacteriological contamination must be considered simultaneously: contamination through runoff water flowing into a drinking water well and contamination of the groundwater.
To assess the risk of water source contamination, an approach based on the travel time for effluent from the latrine to the water source is recommended. To reduce the risk of bacteriological source contamination, the liquid phase coming from the latrine should travel for at least 25 days in the saturated zone of an aquifer. The soil type and the groundwater flow direction must be evaluated. The latter depends on the gradient of the aquifer, which also has a direct influence on the speed at which the groundwater travels.
Water infiltrating from the surface through the unsaturated zone usually flows faster than groundwater in the saturated zone. In figure 5, the water body H1 is higher than the water body H2, meaning that the groundwater will flow from left to right. The hand pump (HP) is most at risk from surface contamination from latrine 2 which has a higher topographic altitude, but most at risk from groundwater contamination from latrine 1 as water is flowing from left to right due to the hydraulic gradient. The hand pump creates a cone of depression in the water table, which can locally invert the flow of water (highlighted in dark blue).
Small amounts of wastewater entering the soil might take a longer time to travel through the unsaturated zone. However, if the unsaturated zone is sufficiently wet, the transport will be several times faster (and the die-off of microbes lower) and so the contamination risk will increase. Therefore the size of the latrine facility and the volume of wastewater potentially entering the soil are important to consider, as well as the impact of rainwater. Large latrine facilities pose a significantly higher risk.
Percolation Test
To assess the speed of movement of contaminated water through the soil, a percolation test should be carried out. Percolation refers to the movement of water through soil, and percolation tests are performed to determine the rate at which water infiltrates. This is an easy test to conduct in field conditions, and gives crucial information when designing a water supply and/or sanitation strategy. There are different methods, each associated with a specific table linking observations to infiltration rates. Percolation tests should be performed in order to check how suitable a site is for projects such as latrines, reservoirs and sanitary landfills.
A percolation test is performed essentially by digging a hole with a shovel or an auger, filling the hole with water to a specified depth and measuring how long it takes the water to drain out of the hole. The base of the test hole should be at the same depth as the planned base of the latrine pits to ensure that the test is a relatively good reflection of percolation conditions at this depth. After the hole is bored or dug and cleaned of loose material, the bottom should be covered with 5 cm of gravel, to avoid clogging during the test. This test should be carried out at the earliest 12 hours after water was first added to the hole (on a wet, saturated soil, not on a dry soil). This procedure must be respected to ensure that the soil is given time to swell and to approach the conditions expected once the sanitation system will be in operation.
The following table gives guideline infiltration rates for clean water and wastewater in different types of soil and simple descriptions to assist soil identification. The soils fall under two broad categories: (1) granular soils, and (2) fissured and fractured soils. It should be noted for granular soils that infiltration rates for wastewater are much lower than those for clean water and are also likely to decrease with time as the soil becomes saturated and clogged. Infiltration also occurs through the walls of the pit, at an angle of about 45°.
| Soil type | Description | Infiltration rate (L/m2/day) or (mm/day) |
|
|---|---|---|---|
| Clean water | Wastewater | ||
| Gravel, coarse, and medium sand | Moist soil will not stick together | 1,500–2,400 | 50 |
| Fine and loamy sand | Moist soil sticks together but will not form a ball | 720–1,500 | 33 |
| Sandy loam and loam | Moist soil forms a ball but still feels gritty when rubbed between fingers | 480–720 | 25 |
| Loam, porous silt loam | Moist soil forms a ball which easily deforms and feels smooth between fingers | 240–480 | 20 |
| Silty clay loam and clay loam | Moist soil forms a strong ball which smears when rubbed but does not go shiny | 120–240 | 10 |
| Clay | Moist soil mold like plasticine and feels very sticky when wet | 24–120 | Unsuitable for soak pits |
For example: if, during the percolation test, the water level drops 12 mm in 30 minutes, this indicates a percolation value (or infiltration rate) in mm/day = 12/30 × 60 × 24 = 576 mm/day (typical value for sandy loam – cf. table 2). Note that the value in mm/day is always equal to the value in L/m2/day. For Soak Pits or Pit Latrines to function correctly, the infiltration rate for clean water should be at least 120 mm/day.
A sanitation system is a multi-step process in which sanitation products such as human excreta and wastewater are managed from the point of generation to the point of use or ultimate disposal. It is a context-specific series of technologies and services for the management of these sanitation products, i.e. for their collection, containment, transport, treatment, transformation, use or disposal. A sanitation system comprises functional groups of technologies that can be selected according to context. By selecting technologies from each applicable functional group, considering the incoming and outgoing products, and the suitability of the technologies in a particular context, a logical, modular sanitation system can be designed. A sanitation system also includes the management and operation and maintenance (O & M) required to ensure that the system functions safely and sustainably. The liquid that has passed through a filter. A mechanical separation process using a porous medium (e.g., cloth, paper, sand bed, or mixed media bed) that captures particulate material and permits the liquid or gaseous fraction to pass through. The size of the pores of the medium determines what is captured and what passes through.The movement of liquid through a filtering medium with the force of gravity. The means of safely collecting and hygienically disposing of excreta and liquid wastes for the protection of public health and the preservation of the quality of public water bodies and, more generally, of the environment. Used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff/stormwater, and any sewer inflow/infiltration.Groundwater Level
The groundwater level can be estimated through the observation of nearby wells, of nearby vegetation (some plants and trees are indicative of high groundwater table)and through interviews with locals. Seasonal variations should also be taken into account, as pits that are dry during the dry season may fill with water during wetter periods of the year. In the worst case, flooding may occur. Groundwater pollution will extend in the direction of groundwater flow (which is mainly horizontal). Therefore, if wells are built in the same aquifer, water should be abstracted from below the polluted zone, provided that the well is adequately sealed at the level of pollution and the abstraction rate is not high enough to draw polluted water into the well. If the pollution of a shallow water table is a cause of concern, it may be necessary to restrict the depth of latrines and use Raised Latrines S.7 or other above-ground solutions. In general, if a water source is being contaminated by a large number of latrines, it is usually easier to move the water source than change the sanitation system. It should be remembered that drinking water contamination also commonly occurs at the point of abstraction, during transport and storage, and at the point of use, through unhygienic collection and storage devices and poor personal hygiene.
A sanitation system is a multi-step process in which sanitation products such as human excreta and wastewater are managed from the point of generation to the point of use or ultimate disposal. It is a context-specific series of technologies and services for the management of these sanitation products, i.e. for their collection, containment, transport, treatment, transformation, use or disposal. A sanitation system comprises functional groups of technologies that can be selected according to context. By selecting technologies from each applicable functional group, considering the incoming and outgoing products, and the suitability of the technologies in a particular context, a logical, modular sanitation system can be designed. A sanitation system also includes the management and operation and maintenance (O & M) required to ensure that the system functions safely and sustainably. An underground layer of permeable rock or sediment (usually gravel or sand) that holds or transmits groundwater. Water that is located beneath the earth’s surface. The means of safely collecting and hygienically disposing of excreta and liquid wastes for the protection of public health and the preservation of the quality of public water bodies and, more generally, of the environment. Used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff/stormwater, and any sewer inflow/infiltration.Mitigation Measures to Reduce the Risk of Microbiological Contamination
If the soil and groundwater assessment show that it is likely that latrines will contaminate a water source, the following options can be considered:
- Implementation of Raised Latrines S.7
- In high water table or flood situations, the containment infrastructure should be watertight to minimise the contamination of groundwater and the environment, with a safe transport of the effluent.
- Surface water sources, such as wells, should be protected to reduce the contamination potential via the ground surface. Protective measures include withdrawing water from a depth below the level of contamination, building a protective well wall at the surface to prevent flood water from entering the well, sealing the well with clay or a similar material to prevent surface run-off from flowing down the side of the well via the annular space.
- Where distances between containment pits and water sources are inadequate, a water safety plan should be implemented to minimise contamination risk.
- Chlorination of drinking water
- Moving the water source
