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Challenging Ground Conditions

3.2 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 groundwater, leading to environmental damage and fatal faecal-oral transmission diseases (e.g. cholera, diarrhea).Contamination occurs due to microbiological agent (bacteria, viruses, protozoans) and chemical (mainly nitrates) entering groundwater from inadequately managed waste. The risk is particularly high when the soil capacity to treat wastewater is low, due to high permeability or short distance to groundwater (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 groundwater 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 faeces” 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 groundwater supplies, which are linked to hydrogeological conditions and existing water supply systems.

Key Actions

To minimize the risk of groundwater contamination from sanitation systems, follow these steps:

(We assume that On-Site Sanitation (OSS) is being installed where groundwater supplies already exist)

(For a more detailed method, see “4.2.2 Installing on-site sanitation alone, where groundwater supplies already exist”, page 49 of the ARGOSS manual).

 

Step 1: Assess contamination risks of existing groundwater 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 groundwater flow direction also has an impact on the travel time.
  • nature of the existing groundwater 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 groundwater supplies.

It should be assessed whether it is possible to ensure sufficient lateral separation between the OSS and the groundwater supply, so that the risk can be considered low. The horizontal separation required is the distance that groundwater 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 aquifer.

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

  1. Vertical distance between the bottom of the OSS pits and the depth of groundwater 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 table according to the SPHERE standards.

  1. 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 groundwater supplies contamination risk remains high with OSS, offsite-sanitation systems are to be assessed, such as sealed pits or small-scale sewerage.

Author(s) (2)
Vincent Dussaux
Claire Papin-Stammose
Solidarité International (SI)
Reviewer(s) / Contributor(s) (1)
Catherine Bourgault
Center for Affordable Water and Sanitation Technology (CAWST)

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