Seawater intrusions in groundwater
Seawater intrusion is the movement of seawater into fresh water aquifers due to natural processes or human activities. Seawater intrusion is caused by decreases in groundwater levels or by rises in seawater levels. When you pump out fresh water rapidly, you lower the height of the freshwater in the aquifer forming a cone of depression. The salt water rises 40 feet for every 1 foot of freshwater depression and forms a cone of ascension. See the following pictures. Intrusion can affect the quality of water not only at the pumping well sites, but also at other well sites, and undeveloped portions of the aquifer.
Use of salt for highways deicing is another source of contamination: when this salt washes off roads it may easily move with percolating water into underground aquifers. An additional problem is created by the fact that piles of salt to be used for deicing have frequently been stored uncovered along roads: rain or snowmelt can dissolve this salt and, though percolation, introduce it into aquifers.
High concentrations of chloride can make water unfit for human consumption and for many industrial uses, but the human health-related problems have not been carefully observed yet. However, high concentrations of sodium ion can contribute to certain heart disease or high blood pressure, particularly in susceptible individuals. High concentration of chlorine has bad effects on the environment as well: it can produce leaf burn and even defoliation in sensitive crops; in lakes can increase the presence of metals in waters and prevent the distribution of oxygen and nutrients and thus harm aquatic life.