Spatial and temporal variability of nitrates concentration in shallow groundwater at the field location of the manure heap

Shallow groundwater is particularly susceptible to contamination by nitrates from agricultural sources. The paper attempts to recognize the direction of spatial and temporal changes in the concentration of nitrates in shallow groundwater, lying under a layer of sandy soil, in the place of a temporary manure storage place and in its immediate vicinity. The study was conducted in the years 2009-2015 on farm focused on milk production, located in the north-western part of the central basin of Biebrza, in Podlaskie voivodeship . The research stand was located at the edge of the field where maize was grown every year and where manure was periodically stored at the turn of 2010/2011 and 2014/2015. Groundwater samples were collected using 7 control and measurement wells, on a monthly basis, 4-8 times a year. The obtained results of nitrates content were subjected to statistical analysis using the analysis of variance with the Kruskal-Wallis test. The analysis of correlation of nitrate nitrogen concentration in water from various research points and analysis of its regression in subsequent years of research was also performed. The average annual concentration of nitrate nitrogen in groundwater samples collected from individual wells was in the wide range from 1.1 to 101.8 mg N-NO3∙dm-3. In the vast majority of cases (86%) it repeatedly exceeded the limit value for the third class of groundwater quality, or 11.3 mg N-NO3∙dm-3. Thus, the tested water should be classified as polluted or at risk of contamination. In addition, in spatial and temporal terms, the average annual nitrates content in the tested groundwater samples collected from wells installed on the research site was characterized by a significant different spatial, has a different intensity as well as directions of changes in time, to a small extent correlated with each other