Using GIS framework and application aWeighted Arithmetic Water Quality Index toassess the quality of groundwater in Bulgaria

The study provides, using GIS-tools, a contemporary assessment of the current state of groundwater quality for domestic purposes on a regional scale in Bulgaria. This is essential for the sound and sustainable management of groundwater resources. In the study, groundwater quality was assessed seasonally and on an annual average basis for the period 2015–2019 by calculating Water Quality Index (WQI) or GWQI (for groundwater). The result presented is derived from the combined effect of several physico-chemical indicators: Hydrogen index (pH), TDS content (total dissolved solids), electrical conductivity (EC), permanganate index (CODMn), total hardness (TH) and ions – calcium (Ca2+), magnesium (Mg2+), sodium (Na+), potassium (K+), chloride (Cl-), sulfate (SO42-), bicarbonate (HCO3-), ammonium (NH4+), nitrate (NO3-). The object of the study is groundwater from the hypergenic zone of the territory of Bulgaria. On average for 2015–2019, groundwater was found to be of excellent and good quality for 87.5% of the country, 8.9% in poor, 1.1% in very poor and only 2.5% unsuitable for drinking purposes (or GWQI > 100). The worst groundwater quality condition is from Sofia-city, where GWQI reaches a maximum value GWQI = 1009 during the spring season (months of April-June) for the period 2015–2019. Significant variation in groundwater conditions is reported in the following districts: Sofia, Pernik, Stara Zagora, Haskovo, Targovishte, Shumen, Dobrich, Varna.