Understanding modern and ancient hydrogeologic features: springs in the Ngorongoro volcanic highland and global wetland facies
Creators: name: Norton, Monica Kelly
Publisher: name: No Publisher Supplied
Year: 2019

description: Groundwater is an important component of the hydrologic cycle and a crucial resource of fresh water for people and animals, as well as crop irrigation. In the subsurface, groundwater is protected from evaporation and climate fluctuations, thereby ensuring persistence and longevity. Surface expression of groundwater discharge (e.g., seeps, spring, and wetlands) supports plants and animals creating ecological niches that leave permanent sedimentary records that vary with biome. Springs and wetlands form under a variety of physical, chemical, and biological conditions. These complexities make understanding how modern systems form as well as interpreting ancient systems in the geologic record a challenge. Precipitation on the Ngorongoro Volcanic Highland (NVH), a ~3000 m high massif of volcanoes situated at the southern bifurcation of the Gregory Rift (the eastern branch of the East African Rift System), provides the water source for several springs surrounding the NVH. Despite the heavy reliance on these springs as a source of year-round fresh drinking water for the many people and animals residing in this region, there have been virtually no studies on the sustainability or longevity of these springs. Insufficient data make groundwater modeling in the region problematic. Therefore, as a step toward generating accurate quantitative groundwater models in this region, a reconnaissance-level, qualitative study was undertaken in Chapter One in which the physical framework of the groundwater basin as well as hydrologic inputs were described by examining nine springs and regional rainfall data. Field and laboratory methods (i.e., site observations of springs, water sample analysis, river flow rate analysis, regional rainfall analysis, and geologic cross-section generation) were used to quantify temporal and spatial rainfall patterns for the study area as well as to generate schematic conceptual models typifying regional modes of spring formation. An analysis of the rainfall data shows that two monsoon-driven wet seasons (October-November "short rains" and March-May "long rains") occur on a yearly basis; peaks in rainfall occur nearly every 5 years and can be partially explained by El Niño and Indian Ocean Dipole events, but other oceanographic factors yet to be determined may also play a role in the interannual variability in rainfall; and the orography of the study area highly impacts the spatial distribution of rainfall (more rain on the NVH than the neighboring plains and rift valley below). Extreme episodic rainfall events, such as the biannual monsoons and interannual years of abundant rainfall seen at the study region, are known to be important for recharging aquifers.Based on geological, geomorphological, hydrological, and geochemical data collected, three schematic conceptual models were developed to capture modes of spring formation for the study region: (1) Groundwater intersects the surface at local base level; (2) Permeable rock (aquifer) intersects surface along slope; and (3) Artesian flow under hydraulic head. These spring settings provide the physical framework for understanding the mechanisms of groundwater supply in the region and a model for what may be occurring in similar rift valley regions. Currently, there is no consensus on a facies model for paleowetlands. Wetlands have yet to be included as depositional environments in classic facies model textbooks. The hypothesis that wetlands have distinct facies in the geologic record and that their facies can be even further distinguished by the mean annual precipitation (MAP) and mean annual temperature (MAT) of their environment is tested in Chapter Two. Modern wetland deposits in the literature from each of the terrestrial biomes of the world, representing varying levels of MAT and MAP, were explored to determine if distinguishing characteristics found in the geologic record set deposits from these biomes apart. While this study determines that there are distinct similarities among wetland deposits across the world, a novel facies model that categorizes wetland deposits into three types by climate (i.e., Tundra/Taiga, Desert, and Grassland/Forest Wetlands) is proposed herein. An improved understanding and identification of the records left by wetlands is crucial for anticipating the future of these diverse yet fragile environments