1. Hydrologic Cycle

• movement and "cycling" of water at a near the Earth's surface; concept of residence time concept of reservoirs- 96% oceans, 3% ice, 1% groundwater, the rest in atmosphere, rivers, lakes, biosphere.
• hydrologic processes- evaporation, precip, runoff, infiltration.
• water as a natural resource- fresh water only is small % of total; mostly from rivers, lakes, and some groundwaters. Water is a "semi-renewable" resource. The amount of precip is the ultimate limit, thus water resources are intimately tied to climate.

2. Hydrology and Climate
Climate involves both temperature and precipitation.  Regional climate strongly controls characteristics of the hydrologic cycle and water supplies.  Relationship between temperature, humidity, and rainfall.  Relationship to local geology/topography (rain shadow effect).
 

3. Groundwater

• huge reservoir of freshwater. (1/4 of all freshwater, the rest is in polar ice). Very valuable but much more delicate resource than surface waters. Longer residence times.
• Aquifers = rocks and unconsolidated materials that store or transmit groundwater.
• porosity - volume% that is empty space, important for storage.
• permeability - ability to transmit fluids; "interconnectedness".

*high permeability is the key for groundwater flow.
• A high quality aquifer is one with both high porosity and permeability.
• Groundwater Table = boundary between unsaturated and saturated zones; elevation of water table mimics the local topography; springs and effluent streams are where WT intersect the ground surface. Recharge/Discharge areas.

GW flows under the force of gravity; from high WT to low WT.

4. Aquifer Types

• unconfined = extends to surface, recharge over large area, water level in aquifer = WT.
• perched = GW ponded above an impermeable layer within a larger unconfined system.
• confined = 'sandwiched' betweeen impermeable layers (aquicludes), water is under pressure, recharge area may be small and far away. Flowing artesian wells- important water resource, no expensive pumping necessary.

• WT elevation is a simple mass balance problem (one-box model):
• recharge = discharge, steady state (no mass loss, equilibrium, WT @constant level).
• discharge > recharge, mass loss, WT drops (ie. drought, rapid pumping).
• discharge < recharge, mass gain, WT risess (ie. wet season, heavy rain).
• Rapid pumping from a single well causes a cone of depression. excessive well field pumping over large areas can causes regional drawdown, compaction and subsidence. (usually related to agricultural use).
• Salt-water intrusion-  shoreline environs; delicate interface between salt and fresh GW (related to density). Excess pumping causes dual cone of depression and salt water intrusion.  Rising sea-level will do the same thing - (climate link)

6. Groundwater Flow Rates: Darcy's Law

• Simple relationship that states that flow velocity is directly proportional to:
• hydraulic gradient: slope of the WT.
• hydraulic conductivity (K): parameter describing the permeability of the aquifer (also depends on the density and viscosity of the fluid).
• Typical rates are on the order of 1-10 cm/day for most aquifers.

 
7. Water Quality

• a measure of water chemistry in terms of trace amounts of particular elements or compounds. Usually measured in ppb or ppm range; Total Dissolved Solids (TDS).

Good quality water usually has TDS <150 ppm; potable up to 1000 ppm.
• Best quality water comes from streams, lakes, reservoirs, and certain GW. EPA sets standards based on medical studies (ie. arsenic in drinking supplies <50 ppb).  Contaminants: non-metals, metals, organics, radioactive materials, organisms.

Contaminant sources: landfills, septic systems, industry, underground tanks, farming.
• Natural GW have variable chemistry dictated by the soil and aquifer composition.
• Cleanest GW comes from qtz sandstone aquifers.
• "Hard" water is due to dissolved Ca and Mg from limestone aquifers.
• Organic-rich soils provide organics and sulfides (sulfur-smelling waters).