Groundwater in the zone of influence of the HPP reservoirs on the lower course of the Sava
Water is a natural resource vital to life on Earth. Based on its mode and location of occurrence, water is classified into two groups: surface water (rivers, lakes, marshes, seas…) and groundwater. Despite being a strategic resource of the 21st century, unfortunately clean water is in seriously short supply around the world today.
Groundwater (colloquially also referred to as underground water) is the water present in aquifers beneath Earth's surface, through which it seeps downward by the force of gravity towards the aquifer's impermeable layer or it moves down the aquifer's sloping surface. Groundwater is fed by surface runoff, which takes the form of rainwater and meltwater as well as water from other sources like watercourses and lakes which seeps through permeable layers (gravel, sand, sandstone) in the ground. Groundwater levels vary throughout the year, depending on the amount of precipitation and the fluctuations in the water levels of the watercourses feeding it – but most of all, groundwater levels vary due to its usage. Wetlands form in areas where the groundwater level reaches the surface or just below the surface.
Figure 1: Diagram showing the groundwater flow and feeding system (Source: https://sl.puntomarinero.com/what-is-groundwater-definition-characterization/)
Groundwater is an essential source of drinking water, providing nearly all of the country's drinking water supply. It is also important for agriculture and fruit farming, where it is directly used for irrigation (pumped from wells, irrigation systems) or where a suitable groundwater level is key to ensuring better conditions for such activities. Groundwater is also used for industrial purposes, for heating and cooling using heat pumps, and more.
Groundwater level monitoring is being carried out in observation wells using piezometers. Monitoring results have shown that groundwater levels are declining as a result of the deepening of stream beds, which in turn causes the drinking water supply to decrease and soil drainage to occur. To reverse the downward trend and meet the strategic goal of securing sufficient amounts of water, certain measures and certain solutions need to be implemented.
Hydroelectric power plants (HPPs), with their reservoirs, are one of the solutions for ensuring a permanent supply of groundwater – thanks to their ability to feed the groundwater and maintain it at a suitable level. This is particularly important in terms of ensuring a safe and secure source of drinking water and favourable conditions for agriculture inside the reservoirs' zone of influence. Suitable groundwater levels also have a positive impact on the restoration of dried-up wetlands, backwaters and streams, as can be seen in the case of HPP Brežice, the last completed hydroelectric power plant in the chain on the lower course of the Sava.
Figure 2: Restoration of a dried-up wetland by the Struga stream (dry prior to the construction of HPP Brežice) at Vrbina, which began flowing again after the impoundment was built, causing the groundwater level to reach the surface (Photo: Krešimir Kvaternik, IBE, d.d.)
Over years the groundwater level slowly somewhat declines. This is primarily caused by a process known as colmatation, where tiny sediments reduce the permeability of the river bed and banks, obstructing the communication between surface water and groundwater.
To maintain a suitable groundwater level and its amounts as a result, groundwater enrichment measures can be taken, which can come in the form of infiltration basins, infiltration wells etc. An infiltration basin was built in the HPP Brežice area, and such structures can also be found abroad – one fine example is an extensive infiltration system of extraction and recharging wells located by the Freudenau HPP reservoir in Vienna.
Figure 3: An infiltration basin for groundwater enrichment in the area of the HPP Brežice reservoir (Photo: Krešimir Kvaternik, IBE, d.d.)
Figure 4: Diagram showing the groundwater enrichment system with extraction and recharging wells located by the Freudenau HPP reservoir in Vienna (Source: https://www.ieahydro.org/media/af466d55/Annex_VIII_CaseStudy1204_Freudenau_Austria.pdf, p. 6)
HESS carries out regular monitoring of the condition of groundwater in the areas of the reservoirs of HPP Boštanj, HPP Arto-Blanca, HPP Krško, HPP Brežice, and the planned HPP Mokrice. To monitor the fluctuation of the groundwater levels there, a large number of observation wells or piezometers were installed.
The construction of hydroelectric power plants with reservoirs on the lower course of the Sava has a positive impact on the state of the groundwater and, in turn, on the quality of living along the Sava by elevating the groundwater levels and increasing the supply of both drinking water and water used for the development of agriculture, fruit farming, and other activities.