The overarching objective of this dissertation was to study groundwater resources on global, local, and regional scales. The first objective of this dissertation was to analyze the groundwater nitrate contamination in the Edwards-Trinity and the Southern High-Plains aquifers of Texas. The second was to study groundwater quality in terms of seawater intrusion in the California Coastal Basin, Upper Floridian, and North Atlantic Coastal Plain aquifers. This dissertation also provided a comprehensive overview of the groundwater level in basins at the global scale and further analyzed agricultural activities on groundwater storage in small and large basins. To achieve first objective, Ordinary Least Square (OLS) and Geographically Weighted Regression (GWR) models were used to study the relationship between groundwater nitrate contamination and land use. This dissertation further identified dominant groundwater types using USGS well data and to estimate the extent of seawater intrusion in terms of dominant ions and ocean salinity in the United States coastal aquifers. Finally, groundwater storage anomaly was quantified using Gravity Recovery and Climate Experiment (GRACE) derived variations in total Terrestrial Water Storage (TWS) and the Global Land Data Assimilation System (GLDAS). Land cover data representing a percentage of irrigated lands using groundwater resources was used to study agricultural activities on groundwater storage. Groundwater nitrate contamination was positively associated with cotton production in Southern High-Plains and Edwards-Trinity aquifers. The nitrate concentrations tended to increase as the well-depth decreased in both aquifers. Results showed that the dominant ions in the study area were Na+ and Cl- . The study concluded that Na-Cl and mixed Ca-Mg-Cl were dominant water types in the United States' coastal aquifers. Results also indicated that seawater intrusion is occurring in the US coastal aquifers. Groundwater depletion has increased in southern Asia, western North America, and southwestern Europe due to groundwater withdrawal for agricultural use. However, farming practice is not the main reason for groundwater scarcity in South America, Africa, and Australia.

Increases in cases of diarrheal disease, respiratory infections and pregnancy complications have been reported in the literature following floods caused by heavy rainfall. Analyzing the association between health records of outcomes related to flooding demarcated by satellite observations will be helpful to evaluate the use of satellite observed products in the mitigation of health risks for future flood events. The Centers for Disease Control and Prevention Social Vulnerability Index (CDC SVI), a relative index assigned to census tracts, measures sociodemographic factors that may affect the ability of communities to prepare for, respond to, and recover from extreme weather events. This index, which quantifies social vulnerability is expected to have a positive relationship with health outcomes associated with flooding. This study uses an inundation map created using observations from active remote sensing satellites to classify census tracts that were flooded after the historic rainfall caused by Hurricane Harvey in Texas in 2017. The duration or period of the inundation was determined using United States Geological Survey (USGS) stream gauge data. A controlled before and after study design was used, and the relative risk (RR) of 11 cause-specific emergency department (ED) visits among the flooded census tracts compared to non-flooded tracts during and after the flood period was modelled using modified Poisson regression while adjusting for a baseline period and the age, ethnicity, race and sex of the patient. Further modification of this relationship by social vulnerability, as measured by CDC SVI quartiles, was examined. The results of this study show that flooding was associated with an increase in ED visits related to carbon monoxide poisoning, insect bites, dehydration, hypothermia, intestinal infectious diseases, and pregnancy complications during the flood period. The average rate of ED visits related to pregnancy complications and insect bites were greater among the flooded tracts compared to the non-flooded tracts in the month following the inundation. Modification of this association by CDC SVI was observed in some cases, such that ED visits were higher or lower in census tracts within higher vulnerable quartiles compared to the least vulnerable quartile. Evaluating the usefulness of earth observations and the CDC SVI in estimating the health risk associated with floods due to Hurricane Harvey has provided understanding the use of these products for future flooding events in identifying specific communities with increased health risks during and following flooding events. / Master of Science / Studies have shown that flooding following heavy rains might increase cases of flood-related health outcomes such as diarrhea, respiratory infections and pregnancy complications among the people in flooded communities. With advancements in satellite technology and image processing, areas that are flooded can be mapped using images captured by satellites within a few days after the flooding. Such maps can then be used to identify communities that might experience greater health risks due to the flooding. This study evaluated the use of such an inundation map created after Hurricane Harvey's floods of 2017 to determine the health risk among the flooded communities with respect to the non-flooded communities. We found that the census tracts that were identified as flooded using the inundation map experienced a greater number of ED visits related to carbon monoxide poisoning, insect bite, dehydration, hypothermia, intestinal infectious diseases, and pregnancy complications compared to the non-flooded census tracts during the period of 19 days after the landfall of Hurricane Harvey. Also, the month following this period, ED visits related to pregnancy complications and insect bites were still greater among the flooded tracts compared to the non-flooded tracts. As the socio-economic status, housing and transportation quality vary among different communities, the association between flooding and the health of the people in different communities may also differ. The Centers for Disease Control and Prevention Social Vulnerability Index (CDC SVI) is a relative vulnerability value assigned to each census tract based on the social characteristics of the population that influence the ability to prevent human suffering and financial loss in a disaster. We analyzed how census tracts grouped as very high, high and moderate vulnerability based on the SVI differ from least vulnerable census tracts in terms of this association. The results show that the association differs between different vulnerability groups for total ED visits and ED visits related to insect bites, intestinal infectious diseases and dehydration. We conclude that the satellite-based products along with the CDC SVI might be useful in identifying communities that might need support to overcome health risks following flooding.