In hydroponics, plants are grown in nutrient-rich water and there is no soil involved in the process. Some water is lost through evaporation, but since it is a closed system, very little water is lost.

Flushing the toilet is the largest contributor to household water consumption, averaging about three gallons of water per flush. 

Seventy-five percent of Earth's surface is water, and the oceans contain ninety-seven percent of Earth’s water. This means that roughly three percent of the Earth's water resides in rivers, lakes, glaciers, and water vapor.

The correct response is littoral. The littoral layer is the layer in a freshwater lake along the shoreline with shallow water. The limnetic layer represent "open water", so this layer is off the coastline and in the middle of the freshwater lake. The benthic layer is along the bottom of the lake (i.e. benthic = bottom). And the profundal layer is the layer at the bottom of deep lakes where the light can't reach the bottom.

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.