HYDROGEOLOGIC FRAMEWORK OF EASTERN NEBRASKA – RECENT DEEPER (EARLY CRETACEOUS TO CENOMANIAN) EXPLORATION

In the past five years, over 25 survey holes have been advanced for partner agencies in eastern Nebraska (enwra.org) seeking new information beyond the typical 4.5 m into bedrock with a primary focus on the Dakota aquifer units. Dakota sediments, up to about 160 m thick, were encountered at depths between 13 m and 344 m (east to west, respectively). Lithologies consisted of interbedded fine sand dominant (~10-30 m thick) and varicolored mudstone dominant (~10-40 m thick) packages, each with varying silt/siltstone content. The heterogeneity, fine grain and non to weak cementation, and drawbacks inherent to the mud rotary technique reduced the level of field confidence in the silt and sand content/character through the faster drilling units. However, downhole natural gamma, spontaneous potential and resistivity data (Century Geophysical, L.L.C. tool and software) validated and/or enhanced the field lithology calls. Comparing four of the 2018 holes (one in the north and three in the west-central portions of eastern Nebraska) with significant identifiable sand packages, the field descriptions were generally similar (grain size, color, composition and diagenetic features and related volumes of material caught/logged with more confidence) but the resistivity values (16 and 64 normal, lateral, single point resistance) were inconsistent. The north packages were generally around 30-35 ohm-m and the west-central were around 20 ohm-m. These downhole resistivity values also generally matched the range of values reported for the airborne electromagnetic survey (AEM) data within 0.55 km of the holes (2015 AEM reconnaissance reports, enwra.org). Additionally, the preliminary top of Dakota stratigraphy calls (~48-215 m depths) are within 10 m of elevations reported for the interpreted AEM data. Monitoring wells were installed in the Dakota at the north and west-central locations for the management districts concerned with quality and quantity (6 m length screens at depths of 116-287 m). Subsequent management collaborations also lead to plans for blending the primary aquifer supply with the Dakota aquifer (~30 m thick package) at the north location. Further regional implications included: a deeper hydrogeologic framework, improved targeting of well screen intervals, mutual enhancement of ground and airborne efforts.