The aim of this study was an assessment of feasibility of conversion of sewage holding (SH) tanks to rainwater harvesting (RWH) tanks in Poland. Such a conversion may partly solve the problem of water scarcity for irrigation of plants in individual small gardens and reduce tap water consumption. Seven methods of RWH tanks sizing were applied to an example of a small harvesting system of the roof area equal to the garden irrigation area of 100 m² for three different irrigation doses. A new criterion was introduced to optimize the tank capacity. Economic optimization was provided for new RWH tanks and for the tanks adapted from abandoned SH tanks. Results obtained for a system sited in west-central Poland in an average year have shown that design capacity of RWH tanks varied markedly between sizing methods. The conversion of SH tanks to RWH tanks is profitable, especially for irrigation due to scarcity of water in relatively dry west-central regions. Conversion of individual SH tanks in a good technical state to RWH tanks is relatively simple and cheap. The potential increase in storage volume due to the conversion of individual SH tanks to RWH tanks could reach all over Poland 215–350 dam³ per year, and individually can save up to 18–25% of total annual water use.

The purpose of this research was to determine the groundwater intrinsic vulnerability to pollution of shallow groundwater in Wielkopolska Province, Poland and to assess the risk of pollution by nitrates. Wielkopolska is known as an area where the problem of water pollution by nitrates has existed for a long time due to intensive agriculture. DRASTIC method and its optimized version as well as four other risk evaluation methods were selected to assess the risk pollution with nitrates. The results of either method did not correlate with nitrate concentrations recorded inthe total of 1679 groundwater monitoring points. Therefore a new method of groundwater pollution risk assessment (NV-L) was proposed. The new method is based on optimized results of the DRASTIC system and the L parameter which considers not only land use types, but also the amount of nitrogen loading leached from soil as a result of fertilizer consumption, and from wet deposition. The final results of NV-L method showed that the largest part of the study area is covered by a very low class of pollution risk (30.6%). The high and very high classes occupy 11.6% of the area, mostly in the areas designated until 2012 as the Nitrate Vulnerable Zones. Validation of the results of all methods showed that the other methods than NV-L cannot be used as a basis for reliable assessment of the risk of groundwater pollution by nitrates, as they do not take into account the nitrogen load leached from the soil profile.