ANALYSIS OF INSECT OCCURRENCE IN GROUNDWATER IN A COASTAL LAKE MICHIGAN AQUIFER

A well field was installed into a shallow (2.5 to 5.7 m below grade) lake-plain aquifer located along the southwestern coast of Michigan, within a mixed residential/urban watershed. The aquifer material is sand to loamy sand. An insect, Folsomia candida, was discovered in the ground water during routine water-level gauging and sampling. The observed organisms are translucent and have no eye parts, indicating a subterranean life-cycle. Organisms from this order (Collembola) are common in soils, but their occurrence solely in ground water, as noted in this investigation, is uncommon. While some insects have been described in the hyporheic zone of a few rivers, little reference is made in the geologic or hydrologic literature to insect occurrence in groundwater.

This presentation reports on attempts to correlate F. candida occurrence and abundance with general ground water parameters such as temperature, pH, major element chemistry, dissolved oxygen, and depth below grade. Soil texture, grain size and shape, organic content, pH and color were also documented. Data collected in November, February and June have yielded preliminary results, including: (1) F. candida generally occur in the same wells throughout the year, and are absent in the same wells throughout the year; (2) F. candida are completely absent in the overlying vadose zone regardless of soil type, based on multiple soil borings/profiles; (3) F. candida are most abundant at a depth of 4.5- 4.9 m below grade; (4) Organism abundance is independent of dissolved oxygen concentration and temperature; and, (5) A moderate negative correlation exists between bicarbonate concentration and F. candida abundance. More detailed chemical and biological studies are planned to evaluate the potential ecological significance of this insect occurrence in groundwater.