Groundwater

Movement of groundwater

The way groundwater moves through the ground is not like an underground river. It moves slowly through soil, sand, and rock. It can collect in areas of gravel, sand, sandstone, or fractured rock called aquifers. The movement of groundwater is slow, at most several feet a day, and depends on how well the spaces between the soil, sand, or rock are connected. The connections between the spaces depend on the porosity and permeability of the ground. Porosity refers to the spaces between the soil, sand, or rock. If an aquifer contains a mixture of sediment sizes and types, such as clay, sand, and gravel, the smaller particles fill the pore spaces between the larger particles and reduce the amount of open space available for water.

Permeability refers to how easily water can move through the pore spaces. If the particles in the ground are all large, as with gravel, all of the pores are large and well connected to one another, so water flows through it fast. In a mix of particles with small sizes, such as clay and sand, and large sizes, such as gravel, the small particles block some of the pore spaces and the water cannot flow as freely.

What’s in groundwater?

Groundwater can contribute nutrients to lakes, but it can also carry them away. Most point sources for nutrients are regulated, such as wastewater, however, there are a lot of unregulated nonpoint sources of nutrients to groundwater that can end up in lakes and cause negative impacts. Some nonpoint sources include soil erosion, agricultural application of fertilizer and manure, and leaky septic systems. Organic nitrogen and phosphorus can occur in dissolved forms that can be picked up by groundwater working its way through the soil and contribute to total nitrogen and total phosphorus in lakes or wells. Highly-permeable soils, with grain sizes similar to sand or larger, can facilitate nutrient movement from the soil into groundwater. This results in the contamination of groundwater which can impact drinking water, lakes, or streams.

For the most part, there is a lot of information and warnings about surface water transfer of phosphorus, such as from over-fertilized fields or runoff from animal droppings on land. There was the belief that phosphorus has low solubility, is strongly adsorbed by soils, and quickly assimilated by plants so it was not transported by groundwater. However, it was more recently found that groundwater is able to carry significant amounts of dissolved phosphorus. This can occur when the bedrock of an area contains large amounts of apatite and may impart higher background phosphorus concentration in the groundwater. It was also found that wetlands, moors, and bogs tend to have higher phosphorus concentrations in their groundwater due to decreased water outflow. In a similar manner, repeat applications of phosphorus-containing fertilizer applied in excess can also lead to phosphorus saturation in soils and an increased risk of orthophosphate transport to groundwater and possibly to a nearby waterbody.

How to protect groundwater?

Groundwater is a hidden resource that is often taken for granted. This puts it at risk of contamination or overuse. Some external methods to reduce nutrient contamination of groundwater include: adding a permeable barrier close to shorelines that can remove dissolve phosphorus from shallow groundwater before it enters the lake, encourage vegetation growth for nutrient uptake, monitoring and fixing leaky septic systems, and decrease phosphorus and nitrogen enriched fertilizer use on agricultural fields. For more tips check out the National Groundwater Association.