**1. Introduction**

The karst phenomena occur in different countries (Central America, North and South China, Slovenia, Estonia, Latvia etc.). The water in a karst aquifer is a major water resource in many regions of some countries [1].

The karst region is located in North Lithuania. Karst processes occur in carbonate rocks (gypsum‐dolomite) forming different surface and underground karst features such as sink‐ holes, caves and caverns. Gypsum is soluble and hence its layers can dissolve and the pro‐ tective cover strata of karst rock become unstable and may subside. In region, gypsum

occurs in contact with water and this causes subsidence problems. In agriculture land, these phenomena are inconvenient, but in urban area, they constitute a geological hazard that can seriously affect development and human safety [2].

The rate of chemical denudation of soluble rocks is one of the main factors determining the intensity of karst process. Gypsum denudation is mainly predetermined by water balance, which depends on meteorological conditions (precipitation and evaporation). The highest intensity of gypsum denudation is during the spring flood and the lowest in the dry period. Since 1978, the intensity of karst denudation has been increased by 30% [2, 3].

The main human activities influencing karst development are groundwater extraction and agriculture [4, 5].

Karst region is densely populated. The two districts (Biržai and Pasvalys) with more than 53,000 of inhabitants [6] are located in it. Soils are fertile; agriculture has been a tradi‐ tional activity on these soils. Precipitation water dissolves not only natural salts from soils but also nutrient surplus from agriculture areas where unbalanced fertilisation rate was applied. Problems are caused by filtration of the water which is rich in salts and which pollutes groundwater. The major amount of precipitation infiltrates into the karst aquifer through Quaternary sediments, however, infiltration takes place through karst sinkholes as well [3, 7]. Karst groundwater becomes polluted frequently and in shorter time periods than water in non‐karstic aquifers [1].

The purpose of this chapter is to present a short review of the research finding on water (drainage, stream, groundwater and sinkhole) quality in agricultural area of North Lithuanian karst region.
