Welcome to Earth Science 2017-2018

Groundwater Chapter 10

EES News and Notes Today, we’re moving on to ground water! We’ll begin with packet work. After that, we’ll move forward through the first section of notes.

Movement and Storage of Groundwater Groundwater reservoirs provide water to streams and wetlands wherever the water table intersects the surface of the ground

The Hydrosphere Water on and in Earth’s crust makes up the hydrosphere. 97% of the hydrosphere is in Earth’s oceans/3% in landmasses. 70% is in glaciers and ice caps; rivers, streams and lakes make up small %.

Groundwater and Precipitation Ultimate source of all water on lands = oceans. Evaporation  precipitation on land Infiltration: precipitation has fallen on land and trickles into the ground to become groundwater.

Groundwater Storage Sandy soils will infiltrate water very quickly. Crust is composed of soil, sediment and rock with porous spaces. Subsurface materials range in porosity of 2% to greater than 50%. Sorting of sands

The Zone of Saturation Region below Earth’s surface in which groundwater completely fills all of the pores of a material. Upper boundary is the water table. Zone of aeration is above WT; filled with air.

Water Movement Water types are either gravitational water or capillary water. Gravitational: trickles down because of gravity. Capillary: drawn upward through capillary action above WT; held in pore space via surface tension.

The Water Table Depth of WT varies by local conditions Stream Valleys: close to surface; shallow Swamps: WT is at Earth’s surface Hilltops: WT is hundreds of meters deep Slope of WT follows topography of land above

Groundwater Movement Groundwater flows downward in slope of WT. This requires flowing through subsurface materials slowly. Permeability: ability of a material to let water pass through it. Large, connected pores promote permeability.

Permeability Groundwater flows in permeable sediment and rock, called aquifers. Fine-grained = low permeability Silt, clay and shale have very fine pores Impermeable layers, aquicludes, impede groundwater flow.

Flow Velocity Flow velocity depends on the slope of the WT and the permeability of the material it moves through Flow velocity is proportional to both slope of WT and permeability of the material water flows in

Springs Groundwater usually emerges where the WT intersects the Earth’s surface, oftentimes at a sloping surface topography. Springs are natural discharges of groundwater.

Emergence of Springs A spring’s discharge might be a trickle or might form a stream. Supersprings, called karst springs, are in limestone regions and discharge from underground paths. Springs also form at edges of perched WTs.

Temperature of Springs Most spring water is the average annual temp of the region it’s located. Groundwater is generally colder in summer and warmer in winter. Some regions discharge hot springs, which have temps above 36.6°C.

Temperature of Springs Most of the hot springs in the US are out west, where there is more igneous activity. Eastern hot springs are due to deep aquifers. Geysers are explosive hot springs; water is boiled, vaporizes and erupts!

Groundwater Weathering and Deposition Chemical weathering of limestone by water causes the characteristic topography of karst areas

Carbonic Acid Most groundwater is slightly acidic due to carbonic acid. CO₂ dissolves in water and combines with H₂O molecules. Limestone consists of calcite; reacts with acids.

Dissolution by Groundwater Carbon dioxide mixes with water to form carbonic acid. Carbonic acid splits into hydrogen ions and bicarbonate ions. Hydrogen ions react with calcite ions Δ calcium ions and bicarbonate ions. CO₂ + H₂O  H₂CO₃ H₂CO₃  H⁺ + HCO₃⁻ CaCO₃ + H⁺  Ca²⁺ + HCO₃⁻

Dissolution by Groundwater Eventually, calcium ions (Ca²⁺) and bicarbonate ions (HCO₃⁻) are carried in groundwater. Eventually, they precipitate or crystallize out of solution. Process of dissolving and precipitation of calcite both play a role in the formation of limestone caves.

Caves A natural underground opening with a connection to Earth’s surface is called a cave or cavern. Most form when groundwater dissolves limestone. Water dissolves, the WT lowers, the cave fills with air and limestone formations form.

Karst Topography Sinkhole: a depression in the ground formed by a collapsed cave or dissolution of limestone by acidic water. ‘Disappearing streams’ form when a surface stream drains into a cave system and keeps flowing; may eventually reemerge as a karst spring.

Karst Topography Limestone regions that have sinkholes and disappearing streams are said to have karst topography. Karst = Croatian region with well-developed features of sinkholes and streams. Sinkholes proliferate and may join to create wide valleys.

Groundwater Deposits Dripstones are remarkable features of rock formations produced by groundwater. Built over time as water drips through caves. Stalactite: hangs from a cave’s ceilings like icicles. Stalagmite: water drops to the floor, building a mound shaped dripstone.

Hard Water Tap water contains various dissolved substances, many of which come from dissolution of minerals in soils and subsurface rock. Hard water can cause problems with heating elements of home appliances and in pipework due to dissolved calcite.

Groundwater Supply Water is not always available in the quantities and in the locations where it is needed and might be compromised by pollution

Wells Wells are holes dug or drilled into the ground to reach an aquifer. Ordinary wells are simple; dug below the WT and replaced by surrounding water. Overpumping occurs when too much water is taken before it can be replaced.

Wells The difference between the original WT level and the level in a pumped well is called the drawdown. If many wells overdraw, shallow wells can run dry. Precipitation water replenishes the water content of an aquifer called recharge. Recharge needs to exceed withdraw to keep wells going.

Artesian Wells An aquifer’s recharge area is often at higher elevation than the rest of the aquifer. An aquifer that contains water under pressure is called an artesian aquifer. With a high rate of recharge, pressurized water can spurt above the land surface to form an artesian well.

Threats to Our Water Supply Freshwater is one of Earth’s most precious resources. Factors such as precipitation, infiltration rates, surface drainage, porosity and volume of groundwater discharge affect estimates of supplies. Changes lead to environmental problems.

Overuse and Subsidence Groundwater supplies can be depleted, decreasing supply and lowering the WT. Ogallala Aquifer When over-pumped, land can subside or sink. Overlying land is heavy, and if water is pumped in the porous space below, the land sinks.

Pollution in Groundwater Most easily polluted groundwater is in WT because of a lack of protection above. Confined aquifers are affected less frequently by local pollution, but can be affected when recharge areas are polluted.

Pollution in Groundwater Sources of pollution include sewage from faulty septic tanks and farms, landfills and other waste disposal sites. Pollutants usually enter the ground above the WT, but they eventually infiltrate into the WT, especially in highly permeable aquifers.

Chemicals Chemicals are dissolved and transported in groundwater. Particles are sub-microscopic in size; easily travel through pores. Once in the aquifer, they are difficult to remove and can contaminate an aquifer easily. EX: Sewage, landfills, waste

Salt Groundwater becomes unusable when salt enters into the water. Predominately a problem in coastal areas. Salt water is denser, but when over-pumping occurs in these areas, underlying salt water rises into the wells, contaminating freshwater.

Radon Radon gas, one of the leading causes of lung cancer in USA, can also pollute groundwater. Found in radioactive decay of uranium in rocks and sed. Found more in granite and shale Often found in basements with poor ventilation.

Protecting Our Water Supply Major polluting sources need to be identified and eliminated. Need to be mindful of runoff and underground pollution. Pollution plumes that exist need to be monitored; sometimes, we can build impervious layers to block spread of pollution. We must be vigilant to save the limited resource!!