The Hydrologic Cycle (with emphasis on groundwater)

Transcription

1 The Hydrologic Cycle (with emphasis on groundwater) The Hydrologic Cycle (viewed in series of steps) 1. Water evaporates from bodies of surface water and from land 2. As moist air is lifted, it cools and condenses to form clouds in atmosphere 3. Once condensed, water falls to ground as precipitation (water or snow)- some can be evaporated back into atmosphere. The Hydrologic Cycle (viewed in series of steps) 4. Water soaks into pores of soil/rock, becoming groundwater, or drawn upward by plants and returned to atmosphere (transpiration) 5. Excess water returning to bodies of standing water as runoff Note that water can pick up various substances at any point in this cycle (water is universal solvent that dissolves and carries many types of substances) So, the net composition of dissolved substances in water reflects the net result of scavenging these substances at various points within the cycle. 1

2 Common sources of water contamination (e.g. trace metal contaminants) 1. Direct dumping of trace metal-contaminated water into bodies of surface water. 2. Leakage of trace metal-contaminated water from mine sites and landfills into surface and groundwater. 3. Emissions from fossil fuel combustion dissolved in water vapour in atmosphere (metals transported in water and fall to Earth in precipitation). 4. Trace metals can be leached from bedrock and transported in groundwater (more so if water is acidified) 5. Trace metals in fertilizers are dissolved in water and find their way into groundwater supply. Surface water contamination is often obvious and acute, and can be remedied relatively quickly if the source of contamination is identified and dealt with effectively (but as in the case of mercury contamination, this is easier said than done) Groundwater contamination is another matter It is difficult to remedy due to a number of geological factors It is important to investigate what groundwater is, and how it behaves. This lecture will focus on aspects of groundwater What is groundwater? Groundwater is water found in the pores of soil and sediment, plus pores and narrow fractures in bedrock Groundwater is the largest reservoir of fresh water that is readily available to humans Importance of groundwater Geological role of groundwater As an erosional agent (forming caverns, sinkholes, etc.) Transportation medium for dissolved substances Groundwater serves as an equalizer of streamflow 2

3 Fresh water of the Hydrosphere Distribution of groundwater Belt of soil moisture water held by molecular attraction on soil particles in the near-surface zone Note that, on average, water resides in groundwater form for a very long time (280 years) Zone of aeration Area above water table Pores occupied by air and water that clings to soil/sediment particles Water cannot be pumped by wells (water held too tightly by particles) Zone of saturation Area below water table Zone where all of the open spaces in sediment and rock are completely filled with water Water within the pores is called groundwater This water can be pumped (flows freely) 3

4 Water table The upper limit of the zone of saturation Distribution of Groundwater Capillary fringe Extends upward from the water table (transitional zone above water table) Note that the level of the water table varies seasonally and year to year (dependent on amount of water that has soaked into ground) Shape is usually a subdued replica of the surface topography (water tends to pile up under topographic highs) Groundwater/Stream Interactions Interaction between groundwater and streams Three types of interactions -Gaining streams gain water from the inflow of groundwater through the streambed -Losing streams lose water to the groundwater system by outflow through the streambed So groundwater is an important regulator of stream flow (streams can give/receive contaminants to/from groundwater Factors influencing the storage and movement of groundwater Porosity percentage of total volume of rock or sediment that consists of pore spaces Determines how much groundwater can be stored Variations can be considerable over short distances 4

5 Factors influencing the storage and movement of groundwater Permeability, aquitards, and aquifers Permeability the ability of a material to transmit a fluid (reflects degree of interconnectedness of pores) Aquitard an impermeable layer that hinders or prevents water movement (such as clay) Aquifer permeable rock strata or sediment that transmits groundwater freely (such as sands and gravels) Movement of groundwater Exceedingly slow typical rate of movement is a few centimetres per day Energy for the movement is provided by the force of gravity Darcy s Law if permeability remains uniform, the velocity of groundwater will increase as the slope of the water table increases (Cam s explanation: groundwater flows fastest where water table is steepest) Movement of groundwater The movement of groundwater is measured directly using Various dyes Carbon-14 Features associated with groundwater Springs Occur where the water table intersects Earth s surface Natural outflow of groundwater Can be caused by an aquitard creating a localized zone of saturation which is called a perched water table 5

6 Springs may result from a perched water table Obtaining groundwater Wells To ensure a continuous supply of water, a well must penetrate below the water table Pumping of wells can cause Drawdown (lowering) of the water table Cone of depression in the water table Formation of a cone of depression in the water table in an unconfined aquifer An artesian well resulting from an inclined (confined) aquifer 6

7 Features associated with groundwater Artesian well a situation in which groundwater under pressure rises above the level of the aquifer Types of artesian wells Nonflowing pressure surface is below ground level Flowing pressure surface is above the ground Not all artesian systems are wells, artesian springs also exist City water systems take advantage of hydraulic head to deliver water to users (in essence, artificial artesian systems) A gushing artesian well in South Dakota Early 1900s- a time when groundwater was plentiful No wells like this now in Western Interior due to excessive withdrawal of groundwater Problems associated with groundwater withdrawal Treating groundwater as a nonrenewable resource - In many places the water available to recharge the aquifer falls significantly short of the amount being withdrawn. - Also, the depletion of groundwater can potentially increase the concentration of contaminants in water (less dilution by water). Subsidence - Ground sinks when water is pumped from wells faster than natural recharge processes can replace it (San Joaquin Valley of California) 7

8 Land Subsidence in San Joaquin Valley, California: 9 metres subsidence total between 1925 and 1975 Problems in road, building construction, and flooding Damage irreversible (and with flooding comes greater surface water contamination from fertilizers, etc.) Problems associated with groundwater withdrawal Saltwater contamination Excessive groundwater withdrawal causes saltwater to be drawn into wells, thus contaminating the freshwater supply This is primarily a problem is coastal areas Saltwater contamination due to excessive well pumping Another problem: sewage One common source is sewage - Sewage can be naturally purified as it passes through a few dozen meters of an aquifer composed of sand or permeable sandstone. - Extremely permeable aquifers, such as coarse gravel, have such large openings that groundwater may travel long distances without being cleaned. - Harmful bacteria (e.g. some strains of E. coli) can cause health problems. - Also, other contaminants, such as trace metals can become concentrated in sewage (remember some trace metals have a tendency to combine with some organic molecules via microbial action). 8

9 Humans can acquire toxic elements directly from water, but also from food. Organisms that we depend upon for food (both plants and animals) can acquire toxic elements from water. So water contamination gives us a double-whammy (can get nasty substances both directly and indirectly from water). Issue of Bioavailability Use of groundwater as source of drinkable water, as well as for irrigation (taken up by crop plants) has increased the availability of dissolved components such as trace metals to humans and other animals. Incidences of groundwater contamination (by dissolved components from both natural and human-linked sources) continue to rise. Proper groundwater management will become increasingly important as groundwater sources become depleted (remember that replenishment of groundwater takes place over centuries to millenia). Think about it. END OF LECTURE 9