Water Quality and Management

-impact on human drinking supply and aquatic habitats

- most contaminants are colorless, odorless, and tasteless

- specific analytical techniques need to be employed- test is only as good as the suite of techniques used

- more tests to be done, more expense

Standards

- water for industrial use can be less pure than drinking water

- In U.S. Safe Drinking Water Act and amendments direct EPA to establish:Maximum Contaminant-Level Goals (MCLGs)Maximum Contaminant Levels (MCLs)

- applies to public supplied water only, not private wells

MCLGs essentially are the targets (e.g. MCLGs for all carcinogens is zero)

MCLs are enforceable limits set as close as feasable to the MCLGs taking into account water treatment technologies and cost.

Primary MCLs are based on health risks, Secondary MCLs are based on asthetics

Risk Assessment

-U.S. EPA uses risk assessment approach to determine required clean-up level for groundwater and soil sites under federal supervision

- e.g. Cancer risk levels are expressed as chance of cancer from 30 year exposure within 70 year lifetime

-assumed person would drink 2 liters from same source every day for 30 years

- if one person in 106 gets cancer at this level, the risk is 10-6

- EPA requires cleanup to the 10-6 risk level

- risk assessment way overestimates risks, and leads to high cost cleanups for exceedingly low risks

- since 25% of pop will eventually get cancer, public accepts cost- but, most cancer caused by smoking and diet

Sources of Ground-Water contamination

Natural Sources: tds, sulfate, and chloride

Human Sources: organic liquids, dissolved organic and inorganic constituents, pathogens

Office of Technology Assessment (OTA) of the U.S. Congress breaks contamination into six categories:

- key contaminant properties for flow are

i) miscibility- extent to which it mixes with water- if miscible and high enough K, contaminant will move with water by advection- if miscible and low K, diffusion will be flow mechanism

ii)density- if miscible, behavior will depend on density to some degree- Some contaminants float on top of saturated zone - some sink through, and may move down gradient along the impermeable layer regardless of groundwater flow direction

iii) solubility- degree to which contaminant dissolves in groundwater

- dissolved contaminants easier to predict flow

iv) volatility- determines extent to which containment will partition into gaseous phase- migration into soil vapor

v) sorption characteristics- degree to which contaminant adsorbs or desorbs from aquifer material

- contaminants may be composed of various compounds making assessment very complex

Contaminant transport

- need to understand mass or solute transport

Transport by Concentration Gradients

- solute moves from high concentration to low

- molecular diffussion or diffusion

- will occur as long as gradient exists, even though fluid is not flowing

recall Fick’s 1st law

where F is the max flux of solute per unit area per unit timeDd is a diffusion coefficientC is the solute concentrationdC/dx is the concentration gradient

- like in Darcy’s Law, negative sign in Fick’s law indicates movement from high to low concentations

- Dd is acquired from tables for different ions

F D dC dxd ( / )

Temperature dependant, about 50% less at 5oC

- where solute concentration is changing with time, Fick’s 2nd law applies:

where ∂C/∂t is the change in concentration over time- diffusion can not proceed as fast in porous media as it can in water- ions must follow indirect path- effective diffusion coefficient:

where ω is a coefficient related to the tortuosity of the medium

Ct D C xd 2 2/

D D d*

- Tortuosity measures effect of flow path shape

- if L is a straight line between the ends of a tortuous flow path of length Le, tortuosity, T=Le/L

- T will always be greater than 1 in a porous media

-T is smaller in well-sorted samples

(T is for demonstration only and not used for calculating omega)

- ω is always less than 1, and is found experimentally- let solute diffuse across medium in lab (

- 0.7 for v. well sorted sand- ranges 0.5 to 0.01 for natural aquifers

- diffusion causes solute to spread away from the place where it is introduced

peecture

- the solute concentrations follow a normal or Gaussian distribution

- based on this distribution, and alternative definition for effective is:

where the numerator is the variance of the distribution

Dtc*

2

2

Sx x

ni2

1

( )

- diffusion complicated by fact that ions must maintain electrical neutrality when diffusing- e.g. in a solution of NaCl, Na+ must diffuse at the same rate as Cl-, unless some other negative ion is available in region to which Na is diffusing

- adsorption to particles may slow diffusion

- possible for diffusion to occur in absence of flow- in low k media, solutes may be moving faster than water

Advection

- dissolved solids carry with water flow, this is advective transport or convection- amount of solute transported = concentration X average linear velocity

recall that average linear velocity is

where ne is the effective porosity

- therefore, advected contaminants move at the average linear velocity

VK

n

dh

d lxe

- the mass flux due to advection is:

advective transport equation is:

solution of this equation yields a sharp concentration front

- situation gives rise to plug flow where all pore fluid is replace with invading solute front

F v n Cx x e

C

tv

C

xx

Mechanical Dispersion

- in reality, ground water particles are moving faster and slower than the average linear velocity

three causes of this

1. fluid will move faster in center of pores than edges2. Some particles will take longer random paths3. Some pores are bigger than others allowing for faster flow

- since all water is not traveling at the same speed, mixing occurs along the flow path- this mechanical dispersion results in a dilution of the solute at the advancing edge of flow

- longitudinal dispersion is in the direction of the flow path (caused by the different speeds)- tranverse dispersion is normal to the flow path

mechanical dispersion = aLvx

where aL is a factor called the dynamic dispersivity of the medium

Hydrodynamic Dispersion

hydrodynamic dispersion takes into account both the mechanical mixing and diffusion

DL=aLvx+D*

where DL is a term called the coefficient of hydrodynamic dispersion

- dispersion is affect by heterogeneity as well- as small k and large k boundaries are encountered, mechanical dispersion will increase

Retardation

- sorption (general term) of solutes in medium will reduce transport- dC/dt = dispersion-advection-sorption+reaction (chemical and biological reactions that change C)- cation exchange (neg clay particles), chemosorption (chemical reaction puts solute on surface), adsorption (adherence to internal walls of particles)