After a 16-m high dam was constructed and a 297-ha pond was arranged in the River Šušvė nearby Angiriai in 1980, the previous natural flow regime of the river has changed: 15500000 m3 of water accumulated above the dam is not discharged via a natural bed, but through two 1.5x1.5 m bottom orifices, and flood discharges (up to Q1% = 296 m3 sE-1) are directed into the lower reach through a 16-m high shaft spillway of floods. Such arrangement of the Angiriai dam ensured it to become an essential obstacle for migrating fish. In 2000, after the arrangement and operation of Angiriai hydro-electric power station, the water regime downstream the dam has been changed and new hydrological pulses as well as ecosystem biodiversity are to be adapted. Thus, the operation of Angiriai HPS on the River Šušvė generating hydroelectric power can have adverse effect on the river habitats and hydro ecological connectivity. To evaluate the effect, the eco-hydraulic water regime measurements in 2005 in a lower stretch with and without turbines operating were made. The study results showed that artificial water levels fluctuation (≈ 4 m hrE-1) induced by Angiriai hydro-electric power station as well as the discharge flowing through the bottom orifices due to high velocities (13–14 m sE-1) and temperature differences could create stressful situations for young fish in the lower bank. According to the data of soil texture of the Šušvė River, the bed armoring process is going on below the Angiriai HPS is quasistable and no impact of HPS operation is found.

Surface water runoff, formed in the company areas due to changing wastewater discharge and pollution fluctuations can be treated in retention ponds. Pollutants, released into the ponds, mineralize during complex natural biochemical processes in the aquatic environment. The paper presents the long-term observation data (2004 – 2016 m.) on the purification of surface wastewater, formed in the company’s production territory, in retention ponds. The aim of this work is to identify surface wastewater purification efficiency in retention ponds. Wastewater samples were taken before and after biological treatment. The samples were investigated in the Chemical Analysis Laboratory of the Water Research Institute of Lithuania University of Agriculture. Biochemical oxygen demand (BOD7 ) was determined with titrometric method, suspended solids (SS) – with gravimetric method, having filtered the substance through a mid-thickness filter. Concentrations of oil pollutants were determined with the help of a spectrophotometric device of infrared rays IKAN-1 in the Analytical Department of Agrochemical Study Center of Lithuanian Agricultural Institute. Although the suspended solids, BOD7, and oil hydrocarbon concentrations, released into the natural environment, were below the MAC when they are collected from the company’s production areas, they are often contaminated; therefore, it is necessary to treat them. The purification efficiency of researched materials was satisfactory and reached 61, 64 and 91%.

The establishment, maintenance and renovation of forest drainage systems are carried out to ensure the development of high quality forest stands and to secure access to forest resources, but it also poses a risk of erosion and increased discharge of suspended solids and nutrients associated with it. The aim of the study was to analyse the efficiency of standard-sized sedimentation ponds built by JSC ‘Latvia’s State Forests’ for sediment and nutrient retention during and after drainage network maintenance. To assess the efficiency of standard-sized sedimentation ponds six study sites were established and various chemical and physical parameters were measured for three years to evaluate water quality and to estimate runoff of plant nutrients and suspended solids. Sampling was carried out before and during drainage network maintenance and during two years following it. Despite the fact that some effect in reducing suspended solid export from catchments was observed, the results revealed insufficient efficiency of the sedimentation ponds in retaining plant nutrients and eroded matter during and after the drainage network maintenance. Retention of Ntot, N-NO3 sup(-) , P-PO4 sup(3-), N-NH4 sup(+) and DOC was ambiguous. It was concluded that other water protection structures should be considered or different parameters of sedimentation ponds should be used to improve the water quality exiting the drainage systems and entering waterbodies.

Establishment of sedimentation ponds is a method to reduce the negative impact of forest management (fellings, drainage) on the quality of nearby rivers, lakes and streams. The aim of the study was to determine the efficiency of sedimentation ponds in forest drainage systems during high precipitation periods. Nine sedimentation ponds with different drainage areas were analysed. The sedimentation ponds are located in the northern part of Latvia on different types of soils and managed by the Joint Stock Company ‘Latvia’s State Forests’. In order to determine efficiency of these ponds the following parameters in water samples were measured: nitrate (NO3 -), ammonium (NH4 +), phosphate ion (PO4 3-), total nitrogen and total suspended solid concentration. The amount of precipitation was taken into account. Results show that there is a correlation between the amount of precipitation and concentrations of total suspended solids and total nitrogen. Ammonium ion concentration was the only parameter that significantly exceeded water quality standards in Latvia. Only one of the objects had high concentration of pollutants, which could be explained with the peaty soil of its drainage area. In further studies data should be collected more frequently, three or four days during one precipitation period.

On the basis of this research and the investigations of other authors (1992 - 2002), the paper presents the comparison of the treatment efficiency of domestic wastewater treatment facilities functioning by different natural means. From all the studied three natural wastewater treatment facilities, constructed wetlands are considered as most efficient for domestic wastewater treatment: here the removal of organic pollutants and biogenic compounds is 85-98% and 67-95% respectively. Biological ponds are most suitable for the treatment of domestic wastewater, the discharge and pollution of which is fluctuating within a wide range. However, such ponds do not always ensure the treatment until the allowable rates. Their treatment efficiency according to biochemical oxygen demand (BOD5) is 49.6%. Total nitrogen amount contained in wastewater outflow from the treatment facilities with biological ponds was on the average 22.2 mg lE-1. The biggest problem of those ponds is rather low P removal efficiency (only 29.5%). Therefore wastewater flowing out of the ponds needs additional treatment. The usage of subsoil filtration fields is limited because their arrangement requires light soils. When arranging the distribution pipeline below the freezing zone, in many places the requirement that the pipes must be 1 m above the maximum ground water level is violated. The pipes should be arranged in places where there is no recipient for treated wastewater outflow. The load of subsoil filtration fields should not exceed 300 m**3 haE-1 per month. From the economical point of view, domestic wastewater treatment facilities with flowing ponds require least expenses.

The paper presents the analysis of the pollution of water and sludge of the pond situated in the park of Kedainiai town. The analysis of pollutants such as heavy metals and oil-products has been carried out with the aim to evaluate the possibilities of arranging a sanitary safe bathing, place in the park and use the accumulated sludge. It was determined that the content of heavy metals in sludge exceeds the maximum permissible limit for sludge pollution in Lithuania. According to cadmium amount, the pollution also exceeds maximum allowable soil pollution level. However, the sludge in the pond contains a rather large amount of oil-products. After carrying out stirring-up tests of bottom sludge, it was determined that turbid water is polluted with oil-products that exceed the maximum permissible limit.