Preliminary study of the impact of individual rainfall event on groundwater levels: taking ‘July 21, 2012 heavy rain’ in Beijing Plain as an example

Torrential rains hit Beijing on July 21, 2012, and provide a good opportunity for the study of the single rainfall event on local groundwater resource. Water level is the macroscopic characteristic of the groundwater system. Through visual comparison of the curves of groundwater level of 618 monitoring wells in the Beijing Plain, it is demonstrated that there are only 34 monitoring wells with an obvious response to the 7/21/2012 heavy rain, accounting for about 6% of the total wells. Those affected are mostly the shallow wells. The effects of the heavy rain to the groundwater levels in the area are not strongly correlated. According to the analysis of the 34 monitoring wells, most of them are located along the upstream channels of the big rivers where the permeability of strata is higher. The occurrence of diffuse stormwater runoff with a longer retention time and more permeable strata are important factors for an obvious response of the groundwater levels. Reasons for the limited influence of a single rainfall are examined. The blocking effects of the clay-type strata, human surface hardening and the compaction of the strata during the sedimentary process, especially the differential compaction, are the three main reasons for this effect. All could reduce both vertical and horizontal permeability; therefore, the effect of the rainfall on groundwater levels could be significantly reduced. The impact on groundwater resources of a single rainfall is generally not very obvious because the quantity of the water is generally not large enough, also the time that the ground remained saturated water is not long enough.