1. Freshwater, Marine and Brackish Water Resources

2. Freshwater pollution

Learning Outcomes

• At the end of this section, the students will be able to – describe difference between different types of

water resources– Identify different sources of freshwater

pollution

Water class Electrical conductivity dS/m

Salt concentration mg/l

Type of water

Non-saline <0.7 <500 Drinking and irrigation water

Slightly saline 0.7 - 2 500-1500 Irrigation water

Moderately saline 2 – 10 1500-7000 Primary drainage water and groundwater

Highly saline 10-25 7000-15 000 Secondary drainage water and groundwater

Very highly saline 25 – 45 15000-35000 Very saline groundwater

Brine >45 >45 000 Seawater

• Freshwater – bodies of water such as ponds, lakes, rivers

and streams containing low concentrations of dissolved salts and other total dissolved solids. In other words, the term excludes seawater and brackish water.

– an important renewable resource– lakes and ponds, rivers, underground

water and man-made freshwater bodies, such as canals, ditches and reservoirs.

• Freshwater is defined – water with < 0.5 ppt dissolved salts

• Water salinity based on dissolved salts in parts per thousand (ppt)– Freshwater < 0.5 – Brackish water 0.5 – 35 – Saline water 35 – 50 – Brine > 50

• Groundwater – water located beneath the ground surface

in soil pore spaces and in aquifers 

• Aquifer – a layer of relatively porous substrate that contains and

transmits groundwater– Unconfined Aquifer - water can flow directly between

the surface and the saturated zone – upper level of saturated layer of an unconfined aquifer

is - water table– Confined aquifer - aquifer overlain by a relatively

impermeable layer of rock or substrate

Problems faced by groundwater utilization

• Overdraft – lowering of water table beyond the reach of existing wells – extraction of groundwater linked to a river system affects river

flows

• Subsidence– hydraulic pressure of groundwater and aquitard (substrate with

relatively low porosity that permits limited transmission of groundwater) supports some of weight of overlying sediments

– by excessive pumping, pore pressures in the aquifer drop and compression of the aquifer causing land subsidence, a drop in the ground surface

– New Orleans, San Jose, Bangkok

• Seawater intrusion – recharge zone of an aquifer near the seacoast

is likely to be inland– a lowered water table may induce sea

water to reverse the flow toward the sea– Sea water moving inland is saltwater intrusion

• Flood mitigation, infrastructure built on floodplains – reduced aquifer recharge

Brackish water

• result from mixing of seawater with fresh water, as in estuaries, or it may occur in brackish fossil aquifers

• Brackish water habitats – Estuaries, mangroves, brackish seas and

lakes–

Uses

– Desalination  for drinking– Aquaculture– In irrigation – moderately salt tolerant/tolerant

crops– Industrial – cooling

Saline water

– three categories• Slightly saline water - 1,000 to 3,000 ppm• Moderately saline water - 3,000 to 10,000 ppm• Highly saline water - 10,000 to 35,000 ppm of salt

– Seawater - roughly 35,000 ppm (= 35 g/L)

Causes of Saline water• Water in rock formations that occurred in marine

conditions • Weathering of salts from rock strata with

naturally high salt content leads to salts moving into aquifers

• Discharge of irrigation return flows • accelerated groundwater seepage to surface

systems• discharges of domestic and industrial

wastewater• intrusion of seawater into coastal aquifers• Over extraction of the freshwater

• Use of Saline water– desalination  for drinking – heated wastewater in the

cooling process and brine disposal issues– agriculture – crop production, aquaculture, forage

production, – in thermoelectric-power industry to cool electricity-

generating equipment– mining and other industrial purposes– used extensively to irrigate gardens, parks and sports

fields– biomass production– salt extraction

• Water Pollution – contamination of water bodies such

as lakes, rivers, oceans, and groundwater  – impaired by anthropogenic contaminants and

either does not support a human use (like serving as drinking water, irrigation water etc.) or limits the support of biotic communities

• Primary sources of water pollution grouped into two – Point-source pollution

• refers to contaminants that enter a waterway through a discrete "point source“ i.e. discharges from a wastewater treatment plant, outfalls from a factory, leaking underground tanks, etc.

– Non-point source pollution• contamination that does not originate from a single discrete

source• cumulative effect of small amounts of contaminants gathered

from a large area i.e nutrient runoff in storm water from sheet flow over an agricultural field