Effects of Soil Properties On Electromagnetic Meter Values.

Surveying soil units with hand tools to find representative sites is labor intensive, time-consuming, and expensive. Yet improper sample site placement may give erroneous data, requiring resampling and possible rejection of development plans or permit release. This study was designed to test the usefulness of a Dualem electromagnetic meter to find representative soils in leveled, apparently uniform wetland soil in Prince William County, VA. The meter is lightweight and practical for field detection of clay and magnetic sand maxima within 1.3 m of the surface. Conductivity zone maps were made by integrating the meter with a global positioning system and data logger, then downloading the data onto a laptop with geospatial software. Over 11,300 transect readings taken over a 12-ha site were grouped into eight conductivity classes. The lowest and highest classes were sampled. Particle-size analysis was conducted on six soils that were described and sampled for map validation. The sand fraction of all soils was separated and tested for magnetic mineral content using a ceramic block magnet (2.2mm x 4.8cm x 9.5mm). Electrical conductivity was negligible in the acid soils. An increase in meter conductivity indicated an increase in clay content and thickness of clayey layer at five of the six locations. The sixth site had a concentric pattern around the highest class. The soil contained lower clay but high amounts of black minerals. However, the magnetic sand was not significantly different between the six sites. Magnetic sand content was found to be inversely related to conductivity value. The electromagnetic sampling allowed for reconnaissance mapping of 12 ha in a few hours of field work. Maps made on-site allowed validation sampling at six points, a saving of time, effort and cost. Targeting large, uniform electromagnetic zones and avoiding unusual map patterns enables proper placement of monitoring wells in wetlands.