Aqueous Flashcards
Characteristics of Chalk
Sedimentary rock comprised of calcium carbonate.
Porous.
Porosity of 40%.
Small pores comprised of shell fragments and bacteria thus withhold water via capillary forces.
1% Specific Yield.
High permeability.
Piper Diagrams Used in Hydrogeology
Portrays how samples can be classified according to setting e.g limestone, alluvial fans, salt flats etc
They also portray ion concentrations/total dissolved solids for samples.
How does chalk influence water movement, storage and quality?
High permeability gives a flat water table.
Cracks/fissures give dual porosity.
Diffusion in pore space induces slow movement. E.g Tritium was found in pore spaces, not flowing rivers.
Piston displacement needed for movement. Requires threshold of at least 4mm/day.
Pores remain saturated but if pores saturate in contaminants this is a problem as it cannot be pumped.
Remediation of a contamination spill. What steps must be taken?
- Identify: Aquifers. Base-flow to rivers. Groundwater Vulnerability. Volumetric flow rate of surrounding water.
- Set up a protection zone. 3 zones - Immediate threat [50 day travel time]. 400 days. Remaining catchment zone.
- Methods can be applied.
Flocculation - coagulation of colloidal particles primarily for metal compounds.
Aeration - circulation of air through the substance.
Trenches/Pipes if shallow - if deep pump water & contaminant using float-controlled pump. Water & oil form an emulsion that a separator must separate. Vapours produced must be pumped via heated air. “Air Sparging” more efficient than water, sir pumped under contaminant rising bubbles carry it to the surface.
Permeable Reactive Barrier [PRB] - artificial barrier/trench reacting with water thus removing pollutants. Long term and cheap. Beware biofueling precipitation and secondary reactants downstream.
Bioremediation in situ - degrade organic material using microbes, fungi, bacteria & protozoa especially on surface of particles. Both aerobic & anaerobic processes.
Most important UK aquifers?
Found in lowlands of England.
Geological Sequence - Younger Cover. Permian - Quaternary.
Chalk -
Permo-Triassic Sandstone - packing of quartz grains imports 30% porosity and 25% specific yield. Soft compact rock, lacking cementation thus groundwater easily flows. Presence of fractures enhances permeability. Yield 5/10 MI/d.
Jurassic Limestone - relatively hard, low specific yield but extensive fracture network enlarged by solution gives high permeability. Yield 30 MI/d.
Lower Greensand - specific yield 10/20%. Moderate permeability. Older lithology thus subjected to more intensive earth movements hence lower permeability/porosity. Great for local supply.
Role of Isotopes determining origin, age and temperature of natural waters?
Origin
. Small environmental change can produce a readily measurable difference in ratios of H,C,N,U & S.
. Relationship between groundwater/meteor water. H&O isotope compositions can be related directly to trend of meteoric groundwater. Confirms that nearly all groundwater originates from hydrologic cycle of precipitation.
.Isotopic composition of groundwater indicates local precipitation is the origin.
Palaeo temperature
. Seasonal correlations of ‘C & delta 18 O as ‘C strongly influences evaporation/condensation. Thus delta 18 O enriched precipitation is found in warmer areas. Latitudes and continental effects affect this.
.Correlation with ‘C and delta 18 O in precipitation is in glacial ice. Ice cores show alternating layers reflecting seasonal atmospheric temperatures and variations thus allowing past climatic conditions to be reconstructed.
How incursions of seawater into aquifers in coastal areas can occur?
. Seawater has higher mineral content, denser and has higher pressure than fresh water.
. Human activities like groundwater pumping at coastal freshwater supplies increases intrusion.
. Water extraction drops the water level, reducing water pressure.
. Agricultural drainage channels form conduits for seawater to move inland.
. Hurricanes cause intrusion.
. Occurs at depth, below freshwater surface of 40x that of freshwater above sea level.
Why are there problems in maintaining sufficient drinking water supplies on many holiday islands?
Rocks are permeable - sand, lava, coral & limestone.
Freshwater only from rainfall.
Water percolates till seawater, forming a layer on-top of the seawater thus must pump from its upper most layer.
[see pad for diagram]
How can we minimise problems of freshwater on holiday islands?
. Rainwater harvesting, surface catchments and underground. Simple and easy to install. Limited to supply/uncertainty of rainfall.m
. Large infrastructure projects such as dams/reservoirs increase storage.
.Non-potable water sources - seawater, brackish water and waste water. Common in many Pacific Islands.
. Water importation.
.Desalination of seawater to freshwater.
How downwards water movement in recharge areas can change to upwards movement and artesian conditions in valleys?
.Potentiometric Surface - imaginary plane in which reservoir fluid will equalise out to if allowed to flow / pierced with wells.
. If surface is above ground surface, a flowing well/artesian well forms.
. Well data enables us to produce the Potentiometric surface.
Artesian - confined aquifer containing groundwater under positive pressure. Water reaches ground surface naturally from a well.
. Below the Potentiometric surface, water has a higher pressure than the atmosphere. Thus pressure always increases with depth.
Definition of: Aquifer Aquiclude Potentiometric Surface Recharge Area Gaining River Artesian Well
Aquifer - underground layer of water-bearing permeable rock/unconsolidated material from which groundwater can be extracted.
Aquiclude - layer of impermeable rock that acts as a barrier to the flow of water.
Potentiometric Surface - level to which a confined aquifer would flow if the aquifer was pierced with wells. “Imaginary Plane”.
Recharge Area - area in which water infiltrates and moves downwards into the zone of saturation of an aquifer.
Gaining River - river of which gains it’s water from the groundwater.
Artesian Well - vertical borehole in which a well is inserted into the aquifer and groundwater from the confined aquifer flows to the Potentiometric surface, normally above the ground surface therefore forming a natural well at ground surface without the need for pumping.
Definition:
Alkalinity
Charge Balance
Oxygen Demand
Piper Diagram
Alkalinity - concentration if basic anions [ability of water to resist acidification].
Charge Balance - total charge of an aqueous solution must be 0 therefore number of positive and negative charges must be equal.
Oxygen Demand - most important oxidising agent in water is dissolved molecular oxygen. Capacity of organic material to consume oxygen in water.
Piper Diagram - graphical representation of the chemistry of a water sample. Cations. & Anions are shown by separate ternary plots.
