The correct response is delta. This is the region where a river meets another body of water. It is a transition zone. Important ecological and biological processes occur in delta regions. One good example of a delta is the Nile Delta where the Nile River meets the Mediterranean Sea. 

The correct response is watershed. A watershed is the drainage basin for a particular region. It’s where water collects or drains. It's functions like a funnel. The other answer choices deal with different aspects of water. Estuaries are transition zones between marine and freshwater systems. Deltas are also transition zones where a river dumps sediment into a body of water. And groundwater is not specific to a region - it's just water that is found underneath the soil surface.

The correct response is 97%. The remaining 3% is freshwater. 

 of the Earth's surface is water, but only  is fresh water. Considering just the fresh water, only about  is accessible to humans in the form of lakes, rivers, and other drinking sources. That is only .007% of the Earth's water. This is the basis for the clean water crisis that faces Earth's population.

750 million people in the world do not have access to clean drinking water. The total world population is about 7 billion, meaning every one in nine people do not have access to clean drinking water. 

A point-source discharge is when pollutants are entering the watershed through an identifiable source, such as a drainage pipe from a factory. Water contamination from agricultural runoff is typically not a point-source because in a largely agrarian region, there are many farms contributing to the pollution but no specific point-source from which the pollutants enter the watershed. Agricultural pollutants typically enter the watershed either by leeching from the soil or through above-ground runoff. 

Greywater typically includes all household sources of wastewater, with the exception of toilets. Greywater can be used to irrigate lawns and golf courses but is not safe for drinking.

Seawater is not fit for use in agriculture. An excess of salt in a field results in water being drawn from plant roots due instead of the other way around. Spartans salted the orchards of their enemies to cripple their civilizations; one should not irrigate with seawater if one's intent is to grow crops. 

The United Arab Emirates is most likely to obtain drinking water by way of desalination. Seawater desalination is used primarily in arid, coastal regions.

The correct choice is "intrusion" because it is the only term that describes the movement of saltwater into freshwater aquifers. Saltwater intrusion is a major threat to many coastal land managers, because fresh groundwater can become contaminated and unusable for residents in the area.