Mathematic model for calculation of the impacts of decentralised sewage systems on groundwater quality

Within the framework of the Project “Implementation of River Basin Management Plans of Latvia towards good surface water status” (LIFE GOODWATER IP, Project number: LIFE 18 IPE/LV/000014) a logical risk modelling tool has been developed for identification and characterisation of the potential impacts of decentralised sewage systems, as well as identifying the risks of environmental pollution using the “traffic light principle”. Based on the results of the analysis, the tool provides general recommendations and lists possible actions to prevent/reduce the impact of the risks.

The aim of the logical risk modelling tool is to provide an instrument for policy planning decision makers and representatives of the municipal wastewater sector for informative overview of the current and potential environmental pollution and public health risks, in support of selection of the most appropriate and environmentally friendly decentralised water treatment technologies in a given area, modelling the development/escalation of the situation with regard to pollution risks, identification of areas where connection to centralised wastewater networks is recommended and/or indication of the need to establish such networks in a specific region, identification of areas where in-depth geological research is required, determination of potential drinking water pollution risks, and highlighting the importance and the impact of the tourism flow and seasonality factor on decentralised sewage systems.

The tool is designed in 2 levels, regarding to complexity, based on geographic information systems. The first level is based on basic or mandatory input data on the impact of the parameters characterising the decentralised sewage system (DSS) on the environment/groundwater and surface waterbodies. In the tool requires data on type of decentralised system installation, filtration system ground type, distance to groundwater, adequacy to groundwater level, is the DSS above the neighbours’ DSS (the model calculates automatically), is the DSS located closer than 5 m from the residential house or the DSS, is the DSS located closer than 2 meters from the property boundary, is the DSS located closer than 10 m from the groundwater abstraction site, is the DSS located closer than 15 m to an open drinking water source, is DSS located closer than 3 meters from the bushes of trees, is the DSS closer than 30 meters from stagnant water (lake, pond, sea), is the DSS located closer than 10 meters from running water (stream, river, drainage ditches), method of manufacture, visual inspection and leak proofness, declared population, actual population/number of users, total volume/ flow according to capacity, capacity adequacy, frequency of export per year, volume produced, per month/ m3 (the model calculates automatically), type of use of the building, construction time, technical documentation, possibility to connect to the centralised sewerage system, last date of survey, date of last maintenance, source of drinking water.

The second level envisages in-depth environmental impact research, analysing background data in addition to basic data: terrain characteristics, dominant soil classes, drainage systems network, data on the hydrogeology of the territory and the structure of vegetation.

Mathematic model is developed for different user groups:

  1. viewer, wear the map view is in clusters and does not allow to identify each individual system on the map, but provides review options on DSS risks and information on request about some of the mandatory data;

2. authenticated user – complete overview of each individual DSS risks and impact on the environment, as well information on request about some of the mandatory data. By clicking on DSS in map, view a pop-up window appears with specific recommendation in relation to the risk, potential pollution, current situation on DSS parameters and background effects. And the second important information – the altitude difference within 100 m radius of surrounding DSS;

3. administration panel – data input and manage function.

The mathematical model is available as a free tool for municipalities or their utility sector delegated companies.

For more information, please contact the project manager and environmental expert from NGO Association “Baltic Coast”  Maija Fonteina Kazeka, e-mail: