Research on the content of nitrogen compounds in waters flowing out of small (A=4.4-36.6 km²) agricultural catchments in Lower Silesia has shown their high content of nitrate nitrogen, ranging from 1.69 to 9.43 mg N-NO₃¯, while maximum concentrations often exceeded 11.3 mg N-NO₃¯ (50 mg of NO₃¯·dm^-3), and thus these waters should be considered highly polluted. Total nitrogen content was directly linked to that of nitrate nitrogen and varied, on average, from 3.21 to 16.85 mg of N·dm^-3. As the pressure is of a local nature, the assessment of the real influence of agricultural areas on the content of nitrates in surface waters requires monitoring of small water-courses that directly collect water from these areas. Research shows a positive correlation between the share of agricultural land in the catchment and the concentration of nitrates flowing out of it. The lack of a strong correlation might be caused by two different reasons. These catchments have a relatively similar share of agricultural land, but first of all, the intensity of agriculture is relatively low.
The use of statistical analysis of the data collected in the process of water quality monitoring is significantly limited by the discrepancy between the distribution in the analyzed data collection and normal distribution.
Conducted analysis has shown that the concentration of C_90% is significantly dependent (r=0.8910 and p<0.01) on the maximum concentration (C_max), whereas the correlation coefficient for relationship C_max-10% =f(C_max) r=0.6392 and p>0.01. The analysis of the relationship between characteristic concentrations (C_90%, C_max-10%) and the average concentration (C_s) has shown an inverse proportion. The results of the analysis clearly show that the concentration of C_90% has a stronger correlation with the maximum concentration C_max, whereas the concentration C_max-10% correlates more strongly with the average concentration C_s. The concentration marked here as C_max-10% (similarly to the 90^th percentile) can constitute a good statistical measure for the general assessment (classification) of waters. The concentration C_90% is a more restrictive measure, as its value also takes into account the extreme values that are not taken into account during the application of other statistical measures, and it can be used for classifications requiring high guarantee.