Sanitation in Areas with Sensitive Groundwater Conditions

Groundwater is a very important resource for human l, particularly frequently used as a source of drinking water. But it can be contaminated by unsafe or inadequate sanitation systems The risk is heightened in areas where groundwater is particularly vulnerable (high water table, high soil permeability). So, when designing a wastewater treatment solution, it is essential to choose:

• appropriate locations: at a sufficient distance from groundwater catchment points and taking account of groundwater circulation;
• technical solutions adapted to hydrogeological conditions: dry toilets, or even watertight or raised-latrines.

Relevance/Importance

Unsafe sanitation systems can pollute groundwaterGroundwaterWater that is located beneath the earth’s surface., leading to environmental damage and fatal faecal-oral transmission diseases (e.g. cholera, diarrhea).Contamination occurs due to microbiological agent (bacteriaBacteriaSimple, single cell organisms that are found everywhere on earth. They are essential for maintaining life and performing essential “services”, such as composting, aerobic degradation of waste, and digesting food in human intestines. Some types, however, can be pathogenic and…, viruses, protozoans) and chemical (mainly nitrates) entering groundwaterGroundwaterWater that is located beneath the earth’s surface. from inadequately managed waste. The risk is particularly high when the soil capacity to treat wastewaterWastewaterUsed water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff/stormwater, and any sewer inflow/infiltration. is low, due to high permeability or short distance to groundwaterGroundwaterWater that is located beneath the earth’s surface. (high water table).

Low-income and informal settlements are especially vulnerable, often located in areas with high water tables and where drinking water relies upon untreated groundwaterGroundwaterWater that is located beneath the earth’s surface. supplies like shallow wells or springs. According to WHO, “in 2022, globally, at least 1.7 billion people use a drinking water source contaminated with faecesFaecesRefers to (semi-solid) excrement that is not mixed with urine or water. Depending on diet, each person produces approximately 50–150 kg per year of faecal matter of which about 80 % is water and the remaining solid fraction is mostly composed of organic material. Of the total…” leading to an estimation of approximately 505 000 diarrheal deaths each year.

Thus, it is essential that sanitation systems are designed considering the risks of contaminating groundwaterGroundwaterWater that is located beneath the earth’s surface. supplies, which are linked to hydrogeological conditions and existing water supply systems.

Key Actions

To minimize the risk of groundwaterGroundwaterWater that is located beneath the earth’s surface. contamination from sanitation systems, follow these steps:

(We assume that On-Site SanitationOn-Site SanitationA sanitation system in which excreta and wastewater are collected and stored or treated on the plot where they are generated. (OSS) is being installed where groundwaterGroundwaterWater that is located beneath the earth’s surface. supplies already exist)

(For a more detailed method, see “4.2.2 Installing on-site sanitationOn-Site SanitationA sanitation system in which excreta and wastewater are collected and stored or treated on the plot where they are generated. alone, where groundwaterGroundwaterWater that is located beneath the earth’s surface. supplies already exist”, page 49 of the ARGOSS manual).

Step 1: Assess contamination risks of existing groundwaterGroundwaterWater that is located beneath the earth’s surface. supplies

• depth to water-table: the deeper the water-table is, the more it is protected from contamination. Depth to water-table can be determined by measuring water-levels in open wells, using local knowledge or from water level records.
• nature of the saturated and unsaturated ground layers: travel time of pathogens through the different types of ground layers (travel time through silt or clay will be longer than through coarse sand, fissured rocks, limestone or gravels). The groundwaterGroundwaterWater that is located beneath the earth’s surface. flow direction also has an impact on the travel time.
• nature of the existing groundwaterGroundwaterWater that is located beneath the earth’s surface. supplies: location, design (open well, borehole, protected spring), and depth of screened sections for boreholes are needed information.

Step 2: Sit and design OSS systems

OSS sitting and design will tend to keep the contamination risks at low level, taking into account three parameters:

1. Horizontal distance between OSS and groundwaterGroundwaterWater that is located beneath the earth’s surface. supplies.

It should be assessed whether it is possible to ensure sufficient lateral separation between the OSS and the groundwaterGroundwaterWater that is located beneath the earth’s surface. supply, so that the risk can be considered low. The horizontal separation required is the distance that groundwaterGroundwaterWater that is located beneath the earth’s surface. would travel (horizontally) in a time interval of 25 or 50 days. See Table 4.4 p. 54 of the ARGOSS manual to get an idea of the minimum horizontal separation needed according to the properties of the aquiferAquiferAn underground layer of permeable rock or sediment (usually gravel or sand) that holds or transmits groundwater..

A strict minimum distance of 30 meters is required according to the SPHERE standards.

2. Vertical distance between the bottom of the OSS pits and the depth of groundwaterGroundwaterWater that is located beneath the earth’s surface. catchment.

It is necessary to assess whether the unsaturated ground can provide sufficient attenuation of pathogens (see ARGOSS Table 4.3 p 49).

A strict minimum distance of 1.5 meter is required between the bottom of pits and the groundwater tableGroundwater TableThe level below the earth’s surface which is saturated with water. It corresponds to the level where water is found when a hole is dug or drilled. A groundwater table is not static and can vary by season, year or usage. according to the SPHERE standards.

3. Alternative sanitation technologies : If the attenuation capacity is not sufficient, specific designs are to be considered, such as dry OSS systems (single pit or VIP pit latrine), since pathogens infiltration is correlated to the hydraulic loading of the latrines. See “Water-Based and Dry Sanitation Technologies”, Emersan Compendium, p 25.

If the groundwaterGroundwaterWater that is located beneath the earth’s surface. supplies contamination risk remains high with OSS, offsite-sanitation systems are to be assessed, such as sealed pits or small-scale sewerage.