Presentation on theme: "Aquifer Water Quality Groundwater Hydraulics Daene C. McKinney."— Presentation transcript:
Aquifer Water Quality Groundwater 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
Sampling and Analysis Plan Document written in advance of sampling that defines: – Sampling locations and frequency – How field parameters are measured – How samples are collected – Quality control and assurance measures Do NOT go to the field without a plan!
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 Ions Based on Water pH Toxic metals less available in water at pH 6 to 8.
Alkalinity acid neutralizing capacity Important because it buffers the water against changes in pH For most waters, alkalinity includes the bicarbonate ion (HCO 3 - ) Other ions such as orthophosphate (HPO 4 - ), borates, may contribute to alkalinity but in small amounts
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 25 o 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 WaterDissolved salt content (mg/l) Fresh water< 1,000 mg/l Brackish water1,000 - 3,000 mg/l Moderatly saline water 3,000 - 10,000 mg/l Highly saline water10,000 - 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 90 o to the direction of light beam to estimate scattered rather than absorbed light Good correlation with concentration of particles in water Thanks to Phil Brown YSI 556 MPS HF Scientific MicroTPI – Turbidity Meter
Water Uses UseTypical quality parameters Public Water SupplyTurbidity, TDS, inorganic and organic compounds, microbes Water contact recreationTurbidity, bacteria, toxic compounds Fish propagation and wildlifeDO, chlorinated organic compounds Industrial water supplySuspended and dissolved constituents Agricultural water supplySodium, TDS Shellfish harvestingDO, bacteria
Abundance of Dissolved Constituents in Surface and Ground Water Major Constituents (> 5 mg/L) Ca Mg Na Cl Si SO 4 2- - sulfate H 2 CO 3 - carbonic acid HCO 3 - - bicarbonate Minor Constituents (0.01-10 mg/L) B K F Sr Fe CO 3 2- - carbonate NO 3 - - nitrate
Abundance of Dissolved Constituents in Surface and Ground Water Trace Constituents (< 0.1 mg/l) Al As Ba Br Cd Co Cu Pb Mn Ni Se Ag Zn 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 – Multiply atomic weight by # of atoms and add together E.g., – Formula weight of water H 2 O = 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 Parameter Sandstone Aquifer mg/LMeq/L Na + 190.827 Cl - 130.367 SO 4 2- 70.146 Ca 2+ 884,391 Mg 2+ 7.30.6 HCO 3 - 3205.245 Total Anions5.758 Total Cations 5.818 % Difference1% 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.
Pie Diagrams Calcium bicarbonate water Magnesium bicarbonate water Sodium chloride water Sodium-calcium bicarbonate water with nitrates
Average Composition of Sea Water and Mississippi River water Parameter Sea water (mg/L) Mississippi River water (mg/L) Na10,50020 Cl19,00024 SO 4 2,70051 Ca41038 Mg39010 HCO 3 142113
Ground Water Quality in Different Aquifers Parameter Sandstone Aquifer Limestone Aquifer Igneous/ Volcanic Aquifer Shale with Salts Alluvium (Farmland) pH126.96.36.199.17.4 Na19291841220114 Cl13536198030 SO 4 7607100074 Ca881443435364 Mg7.35524215919 HCO 3 3206221,300355402 NO 3 0.40.30.22.460
Aquatic Freshwater Protection Criteria (USA EPA Guidelines) CriteriaRecommended Standard pH6.5-9.5 Alkalinity20 mg/L or more Dissolved Oxygen 30 day average 5.5 mg/L (warm water fish) Suspended Solids Should not reduce Photosynthesis by more than 10% in the water
Drinking Water Criteria (USA EPA Guidelines) CriteriaRecommended StandardReason Coliform Bacteria0 colonies/mlHealth pH6.5-8.5Aesthetic Barium2 mg/L Health Nitrate10 mg/LHealth Total Dissolved Solids 500 mg/LTaste
Groundwater Sampling Important Points – Be sure to take a representative sample – Make sure sample bottles are properly rinsed – Filter and preserve samples in the field – Take field measurements with proper equipment – Store on ice – Send to a certified water chemistry laboratory within 24 hours of sampling – Have a quality control program with duplicates, blanks, field blanks, or spiked samples
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)
Sodium and Irrigation Low-sodium water – Used on all soils with little danger of harmful levels of exchangeable sodium. Medium-sodium water – appreciable sodium hazard in certain fine-textured soils High-sodium water – harmful levels of exchangeable sodium in most soils – require special soil management such as good drainage, leaching, and additions of organic matter. Very high sodium water – unsatisfactory for irrigation unless special action is taken, such as addition of gypsum to the soil
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 USGS analyzed US water quality data 10 ppb level exceeded by 8% of public ground waters tested EPA estimates that the 10 ppb rule affects about 4,000 water systems "Hotspots" of high concentration – Central New England – Midwest – Western states.
Chemistry Arsenic has the ability to switch between two valency forms, – As 3+ and As 5+. As 3+ – more soluble and more likely to be absorbed than As 5+ – This switching property makes detection and measurement difficult and frequently unreliable
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