Aquifer Water QualityGroundwater Hydraulics

Daene C. McKinney

Introduction• Groundwater Quality

– Sampling Plan– Field Measured Parameters

• pH• Alkalinity• Conductance• Salinity• Dissolved Oxygen• Turbidity

– Chemical Equivalence– Laboratory QA/QC– Diagrams

• Piper• Stiff

– Water Quality Classification– Irrigation Water

• Sodium• Salinity

– Arsenic– Iron Bacteria

Water Quality Management Process

• Identify – Problem– Indicators – Target Values

• Assess source(s)• Determine linkages

– Sources Targets

• Allocate permissible loads

• Monitor and evaluate• Implement

Groundwater Quality

• Helps us understand the hydrogeologic system• Indicates comingling of groundwater and

surface water• Helps us interpret groundwater flow dynamics• Delineates groundwater contamination

Basic Water Quality Parameters

• pH• Specific conductance (EC)• Salinity• Total dissolved solids (TDS)• Turbidity • Dissolved oxygen (DO)• Biochemical oxygen demand (BOD)• Temperature

pH

• Measures hydrogen ion concentration

• Negative log of hydrogen ion concentration

• Ranges from 0 to 14 std. units • pH

– 7 neutral– 0 - 7 acidic – 7 - 14 alkaline

Thanks to Phil Brown

Solubility of Specific IonsBased on Water pH

Toxic metals less available in water at pH 6 to 8.

Conductivity

• Measures electric conductivity (EC) of water

• Higher value means water is a better electrical conductor

• Increases when more salt (e.g., sodium chloride) is dissolved in water

• Indirect measure of salinity• Units are μmhos/cm at 25o

C or μsiemens/cm

Thanks to Phil Brown

Conductivity at Barton Springs• Specific conductance is an indication of the hardness of water. The specific

conductance declines in spring water when rainfall enters the aquifer and later discharges in the spring. Below is a graph demonstrating this effect in Barton Springs. Rainfall is indicated in red, and specific conductance in blue.

Salinity

• Classification of Ground Water• Composition Based on Total Dissolved

Solids Content

Salts in Sea Water

Type of Water Dissolved salt content (mg/l)

Fresh water < 1,000 mg/l

Brackish water 1,000 - 3,000 mg/l

Moderatly saline water

3,000 - 10,000 mg/l

Highly saline water 10,000 - 35,000 mg/l

Sea water > 35,000 mg/l

Dissolved Oxygen

• Amount of gaseous oxygen (O2) dissolved in water

• Oxygen gets into water by diffusion from the surrounding air, by aeration, and through photosynthesis

• DO range from 0-18 mg/l• Need 5-6 mg/l to support a

diverse population• DO < 2 mg/l - Hypoxia

Thanks to Phil Brown

Turbidity• Measured in Nephelometric

Turbidity Units (NTU)• Estimates light scattering by

suspended particles• Photocell set at 90o to the direction

of light beam to estimate scattered rather than absorbed light

• Good correlation with concentration of particles in water

Thanks to Phil BrownYSI 556 MPS

HF Scientific MicroTPI – Turbidity Meter

Water UsesUse Typical quality parameters

Public Water Supply Turbidity, TDS, inorganic and organic compounds, microbes

Water contact recreation Turbidity, bacteria, toxic compounds

Fish propagation and wildlife DO, chlorinated organic compounds

Industrial water supply Suspended and dissolved constituents

Agricultural water supply Sodium, TDS

Shellfish harvesting DO, bacteria

Abundance of Dissolved Constituents in Surface and Ground Water

Major Constituents (> 5 mg/L) CaMgNaClSiSO4

2- - sulfateH2CO3 - carbonic acidHCO3

- - bicarbonate

Minor Constituents (0.01-10 mg/L)BKFSrFeCO3

2- - carbonateNO3

- - nitrate

Abundance of Dissolved Constituents in Surface and Ground Water

Trace Constituents (< 0.1 mg/l)AlAsBaBrCdCoCu

PbMnNiSeAgZn others

Water Classification

• How?– Compare ions with ions using chemical equivalence– Making sure anions and cations balance– Use of diagrams and models

• Why?– Helps define origin of the water – Indicates residence time in the aquifer– Aids in defining the hydrogeology– Defines suitability

What is Chemical Equivalence?

• Chemical analysis of groundwater samples– Concentrations of ions are reported by

• weight (mg/L) • chemical equivalence (meq/L)

• Takes into account ionic charge• Equivalent Concentration

Formula weight• Formula weight

– Multiply atomic weight by # of atoms and add together

• E.g., – Formula weight of water

H2O = 2*(Atomic Wt of H) + 1*(Atomic Wt of O)

2*(1.008) + 1*(16) = 18.01

Atomic Weight (Relative atomic mass) is a dimensionless physical quantity, the ratio of the average mass of atoms of an element to 1/12 of the mass of an atom of carbon-12

Ion Balance

• If all ions are correctly determined by a lab– sum of cations should equal sum of anions (all in

meq/L) • Errors in analysis and chemical reactions in

samples – 5% difference is considered acceptable– > 5%, question the lab results

Calculating Equivalence

ParameterSandstone Aquifer

mg/L Meq/L

Na+ 19 0.827

Cl- 13 0.367

SO42- 7 0.146

Ca2+ 88 4,391

Mg2+ 7.3 0.6

HCO3- 320 5.245

Total Anions 5.758

Total Cations5.818

% Difference 1%

For instance:

The atomic wt. of Sodium (valence of one) = 22.989

And its charge is one

Dividing the concentration of sodium in the sample (19 mg/L) by its “combining wt.” = 0.827 meq/L or its equivalent concentration.

Use of Diagrams

• There numerous types of diagrams on which anions and cations (in Meq/L) can be plotted. These include:– Piper– Stiff– Pie– Schooler– Depth Profile

Stiff Diagrams

• Concentrations of cations are plotted to the left of the vertical axis and anions are plotted to the right (meq/L)

• The points are connected to form a polygon.

• Waters of similar quality have distinctive shapes.

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yk-31

yk-16

yk-27

yk-101

Stiff Diagrams in Cyprus

Average Composition of Sea Water and Mississippi River water

ParameterSea water

(mg/L)Mississippi River

water (mg/L)

Na 10,500 20Cl 19,000 24

SO4 2,700 51Ca 410 38Mg 390 10

HCO3 142 113

Ground Water Quality in Different Aquifers

ParameterSandstone

AquiferLimestone

Aquifer

Igneous/Volcanic Aquifer

Shale with Salts

Alluvium(Farmland)

pH 7.5 7.8 6.5 7.1 7.4

Na 19 29 184 1220 114

Cl 13 53 6 1980 30

SO4 7 60 7 1000 74

Ca 88 144 34 353 64

Mg 7.3 55 242 159 19

HCO3 320 622 1,300 355 402

NO3 0.4 0.3 0.2 2.4 60

Aquatic Freshwater Protection Criteria (USA EPA Guidelines)

Criteria Recommended Standard

pH 6.5-9.5

Alkalinity 20 mg/L or more

Dissolved Oxygen30 day average 5.5 mg/L

(warm water fish)

Suspended SolidsShould not reduce Photosynthesis by

more than 10% in the water

Drinking Water Criteria(USA EPA Guidelines)

Criteria Recommended Standard Reason

Coliform Bacteria 0 colonies/ml Health

pH 6.5-8.5 Aesthetic

Barium 2 mg/LHealth

Nitrate 10 mg/L Health

Total Dissolved Solids

500 mg/L Taste

T07_04_02

Hardness of Water

WELL SAMPLING

• Calculate Well Volume: – Determine static water level– Calculate volume of water in the well casing

• Purge the well: – A minimum of three casing volumes is

recommended.

ANALYSIS OF WATER SAMPLES

• Field:– pH, specific conductance, temperature,

dissolved oxygen, and alkalinity• Laboratory:

– Cations: sodium, calcium magnesium, potassium, and iron

– Anions: bicarbonate, carbonate, sulfate, and chloride

– Trace Metals, Radioactivity

Sodium and Irrigation

• Sodium reacts with soil to reduce permeability.• Alkali soils - High sodium with carbonate• Saline soils – High sodium with chloride or sulphate• Neither support plant growth• Sodium Adsorption Ratio (SAR)

Salinity and irrigation

• Low salinity water – used for most crops

• Medium salinity water – used with moderate amount of leaching (potatoes, corn,

wheat, oats, and alfalfa) • High salinity water

– Cannot be used on soils having restricted drainage. • Very high salinity water

– Can be used only on certain crops and then only if special practices are followed

Arsenic in Groundwater

• Long-term exposure to arsenic from drinking water is directly linked to:– Cancer of the skin, lungs, urinary bladder and kidneys.– Acute gastrointestinal and cardiac damage as well as

vascular disorders such as blackfoot disease. – Sub-lethal effects include diabetes, keratosis, heart

disease and high blood pressure.• Toxicity is dependent on diet and health, but is

cumulative. Arsenic is excreted very slowly by the body through deposition in the hair and nails.

BACKGROUND

• Arsenic (As) – toxic metal widespread in groundwater

• Occurs widely in aquifers – deltaic sediments near mountain uplift zones– deep sandy aquifer layers originating as riverine,

lake or coastal deposits. – Ganges, Mekong and Red River deltas, sandy alluvial

deposits in South Asia, South East Asia, South America, and in many parts of North America and Europe.

Arsenic Contamination

• Associated with fluctuating water tables and flooding cycles particularly in – Acidic sulfate soils or – Iron and/or manganese-enriched layers, – saline-layered aquifers

• Levels in water supplies can vary through a year adding to the difficulties of identification and monitoring.

Drinking Water Standards

• Worldwide 50 ppb limit (1942) • US EPA

– Acceptable mortality = 1 death per 1,000 people for carcinogens

– Lifetime risk from exposure to 50 ppb As • 13 cancer-related deaths per year per 1000 people

(1992)

– Current standard = 10 ppb standard

Arsenic in the United States

Summary• Sampling Plan• Field Measured Parameters

– pH– Alkalinity– Conductance– Salinity– Dissolved Oxygen– Turbidity

• Chemical Equivalence• Laboratory QA/QC• Diagrams

– Piper– Stiff

• Water Quality Classification• Irrigation Water

– Sodium– Salinity

• Arsenic