Jelgava is 4th largest town in central Latvia with 60 thousand inhabitants. Centralised water supply of the town completely depends on groundwater. There is one main wellfield Tetele with 12 wells installed in the confined aquifer. Aquifer consists of upper Devonian sandstone with siltstone and clay interlayers lying at the depth of 156-260 meters from Earth’s surface. The well discharge varies between 1300-2100 m3/day, and the total planned discharge of the wellfield is 17.4 thousand m3/day. Static water level in the wells is 3-4 m above Earth’s surface (9.6 m amsl), and the drawdown in the wells is 11-23 meters. Daily 10 wells are in operation and two are kept as a reserve. Some decrease of well discharge had been observed therefore assessment of current well discharge and well loses as well as wellfield discharge and forecast of discharge decrease in time was needed.
Finite element transient 2D hydrogeological model was used to calculate water level changes in the wellfield area due to well discharge, well loss and well interference. Hydraulic model was used to calculate pressure loss in the water pipelines and their impact on well discharge decrease depending on piezometric level.
The hydrogeological model was calibrated on water level fluctuations in wells depending on well discharge regime using long-term SCADA data of the wellfield. The hydrogeological modelling yielded high accuracy results providing data on aquifer transmissivity and leakance, storage of sandstones and well loses and their changes in years 2014-2016.
Fluctuations of piezometric level in the aquifer and wells as well as wellfield discharge were simulated taking into account changes in well loses, well interference, technical parameters of submersible pumps and pressure loses in the water pipelines. Constant increase of well loss in two wells was found resulting in continuous decrease of well discharge. On the other hand, well loss in those wells correlated with the amount of water abstracted. Changes of well loses and related decrease of wellfield discharge in time were quantified.
It was concluded that wellfield discharge will decrease from 17 thousand m3/day today to 14.4 thousand m3/day in 10 years in the worst scenario. Some measures to postpone this some mitigation measures like similar discharge of all wells and other were recommended.