Infiltration should be used wherever possible, subject to protection of groundwater against polluted runoff. As all runoff is to some degree contaminated, protection is provided by ensuring that some treatment is provided by SuDS systems and that the base of all infiltration units should be at least 1.0m above the highest expected groundwater level. This is often difficult to establish, so groundwater levels should be measured through at least one winter period and a judgement made as to how much higher it might get in a really wet winter.
Where there are important aquifers, or the runoff is particularly contaminated, there may be a need to prevent the use of infiltration, even where the infiltration rate is low. The SuDS Manual and other guidance should be followed where this situation applies.