Total Carbonate Hardness in Cumberland Valley Groundwater

A Shippensburg University Practical Exam

Dana Heston

3/6/2015

1

TABLE OF CONTENTS

INTRODUCTION 2

LITERATURE REVIEW 3

Hard water 3

Impacts on Health 5

Treatment Options 6

Specific Conductance 7

STUDY AREA 7

Geology 8

METHODS 10

Cumberland and Franklin County Data 11

Big Spring 12

RESULTS 13

Field Sites 13

Conductance and Hardness 14

DISCUSSION 15

Outliers 17

Specific Capacity and Sustained Yield 18

CONCLUSIONS 18

REFERENCES 19

TABLES, EQUATIONS, AND FIGURES

Figure 1 8

Figure 2 9

Figure 3 15

Table 1 4

Table 2 14

Equation 1 3

Equation 2 12

2

INTRODUCTION

Hard water is water that contains a high concentration of multivalent cations,

commonly calcium (Ca) and magnesium (Mg) (Evens et al., 2013). These ions react with

soap, preventing it from forming lather (Evens et al., 2013). Hard water is generally not

considered to be harmful to human health, but makes cleaning difficult and leaves a

residue called lime scale on surfaces. Hard water can be difficult to drink because of the

unpleasant taste associated with the excessive mineral content.

Geology plays an important role in the hardness of groundwater. Water becomes

hard when mineral dissolution occurs as the flowing water comes in contact with rock

and soil. Groundwater is generally harder than surface water and hardness increases as

a function of the distance of the flowpath (Becher and Root, 1981). The longer the water

is exposed to soluble bedrock, the harder the water will be when it is pumped for public

or domestic use. Water molecules surround the disassociated Ca and Mg ions,

preventing them from recombining (Meena et al., 2012). Water flowing through

gypsum, limestone, and dolostone bedrock can be predicted to be hard. Conversely,

water flowing through bedrock high in silica is less likely to undergo extensive mineral

dissolution.

The intention of this study is to answer the following questions regarding existing

techniques used to calculate water hardness and the relationship between specific

conductance (SpC) and hardness:

Becher and Root showed a relationship between SpC and hardness that can aid in

determining the concentration of calcium carbonate (CaCO3) in a water sample

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by multiplying the SpC by 0.48. Is this an accurate method for calculating water

hardness?

When additional lab data are available the concentrations of Ca and Mg can be

used to calculate CaCO3 concentrations. Will using this method show a

relationship between SpC and hardness similar to the one identified by Becher

and Root (1981)?

There are several outliers in the data, evident on the scatterplot. Why are these

data points not displaying the same trend as the majority of the data? Are these

water samples unique, or are data entry errors affecting the analysis?

LITERATURE REVIEW

Hard Water

Polyvalent, metallic ions dissolved from rock and soil cause water to become

hard. Calcium (Ca) and magnesium (Mg) are the most common ions even though any

positively charged divalent ions, such as iron (Fe), strontium (Sr) and manganese (Mn)

can cause hard water (Meena et al., 2012). The Ca and Mg react with bicarbonate

(HCO3) when water is heated to form an insoluble precipitate. The precipitate left

behind is referred to as scale and leaves a residue inside pipes and on surfaces as water

evaporates. The chemical reaction of hard water to scale is shown in equation 1 (Casiday

and Frey, 1998).

2233

2 )()(2)( COOHsCaCOaqHCOaqCa

Equation 1: The formation of scale when hard water is heated (Casiday and Frey, 1998).

4

Water hardens as it flows through bedrock such as limestone, dolostone, marble,

and gypsum. High concentrations of Ca and Mg in the bedrock quickly add these ions to

the water as dissolution occurs. High concentrations of CO2 in soil and subsurface

voids increase the amount of carbonic acid (H2CO3) in groundwater, which increases

the rate of mineral dissolution. Carbon dioxide (CO2) is heavier than oxygen (O2) and

is found in stronger concentrations at increased depths (Atkinson, 1977). Subsurface

gases diffuse from high pressure to low pressure environments, such as voids. Caves

and subsurface channels in karst landscapes often have elevated levels of CO2, as well as

the fractures leading from these features due to the migration of CO2 into the void

spaces. The stronger concentration of H2CO3 increases the rate of mineral dissolution

allowing water to become hard even though conduit flow is likely in this environment.

Water hardness is also increased in areas with increased calcium carbonate (CaCO3) in

the soil and high concentrations of CO2 in the unsaturated zone (Atkinson, 1977). Table

1 shows the scale used by the Environmental Protection Agency (EPA) to determine the

hardness of water by calculating the amount of CaCO3 (1986).

Table 1: Environmental Protection Agency (EPA) hardness scale for concentrations of calcium carbonate in water (EPA, 1986).

Classification CaCO3 in milligrams per liter (mg/L)

Soft 0-75

Moderately Hard 75-150

Hard 150-300

Very Hard 300 and up

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Impacts on Health

The EPA has not set a maximum contaminant level (MCL) for hard water.

Calcium and Mg are essential for human health in adequate quantities and hard water

can contribute to these dietary needs. Research indicates that consumption of hard

water can actually improve health if the ratio of Ca to Mg is within an acceptable range.

The concentration of Mg is important in determining the effect hard water can have on

human health (Evens et al., 2013). Total hardness of water with a concentration greater

than 200 milligrams per liter (mg/L) with a Mg concentration less than 7 mg/L can

cause adverse effects on human health (Evens et al., 2013).

Calcium is more abundant than Mg in the environment and consuming large

quantities can be harmful. Magnesium acts as a natural calcium antagonist by

competing for binding sites in vascular muscle (Evens et al., 2013). The Mg content

must be adequate to control the amount of Ca absorbed. A study conducted in Sweden

indicated that a high Ca intake could lower s-cholesterol and s-LDL, but increased

systolic blood pressure (SBP) (Nerbrand et al., 2003). Epidemiological evidence

suggests that Mg plays an important role in regulating blood pressure (Anne, 2011).

Drinking water with 10-100 parts per million (ppm) Mg could potentially prevent 4.5

million hearth disease and stroke deaths per year, worldwide (Rosanoff, 2013).

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Treatment Options

There are different treatment options available to soften hard water. Reducing

the temperature of a water heater can help to reduce the amount of scale left behind by

reducing the amount of precipitate formed. This is the most economical treatment

option.

Cation exchange is a popular option for softening domestic water supplies. The

Ca and Mg ions in the hard water are exchanged with sodium (Na) or potassium (K).

The cation exchange technique can remove nearly all of the Ca and Mg and even 5-10

ppm of Fe and Mn (Skipton and Dvorak, 2014). However, the cation exchange method

increases the Na concentration in water which can have adverse effects on human health

if consumed in large quantities. Cation exchange systems can be installed to soften only

water going into the water heater to alleviate the concern of increased sodium intake

from ingesting softened water from cooking and drinking.

Reverse osmosis forces water through a filter to remove minerals and other

contaminants. This technique avoids the use of salt but very hard water can clog the

filter making it an expensive alternative in areas of very hard water (Cool Today, 2015).

It is possible to use a water softener in conjunction with a reverse osmosis filtration

system to extend the life of the filters in hard water areas.

Large municipal supplies will often use a lime-soda process for ion exchange.

Lime and soda ash are added to the water, combining with the Ca and Mg. The Ca

precipitates as calcium carbonate (CaCO3), and the Mg as magnesium hydroxide

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(Mg(OH)2) (Casiday and Frey, 1998). The precipitates are then removed through

filtering.

Specific Conductance

Electrical conductivity is a measure of charged ions in the water. Pure water is a

poor conductor of electricity until ions are dissolved. This parameter is temperature

dependent and specific conductivity (SpC) is reported in most studies because it is

normalized to 25 degrees Celsius.

The Ca and Mg ions that are reported as total hardness contribute to SpC

concentrations. Clean limestone waters typically have a total hardness value of close to

half of the SpC (Krawczyk and Ford, 2006). This is confirmed in the formula of 0.48 *

SpC given in Becher and Root (1981). Specific conductance values can also be impacted

by the presence of ions such as K, Na, Cl, NO3, and SO4 that are dissolved in water from

both natural and anthropogenic sources (Krawczyk and Ford, 2006).

STUDY AREA

The study area includes Franklin and Cumberland counties in south-central

Pennsylvania, part of the Cumberland Valley portion of the Great Valley. The

Cumberland Valley lies between South Mountain and Blue Mountain in the Ridge and

Valley Physiographic Province (figure 1). The structural geology of the study area is

complex with deformation in the rock formations common due to the Taconic, Acadian,

and Alleghenian Orogenies (mountain building events) that formed the Appalachian

Mountains.

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Figure 1: The Cumberland Valley study area lies between Blue Mountain and South Mountain in south-central Pennsylvania.

Geology

The stratigraphic column (figure 2) shows the geology of the study area which

spans the colluvium deposited at the base of South Mountain to the Lower Cambrian

Tomstown Formation. The northwestern portion of the study area is comprised of the

Martinsburg Formation, which contains dark-gray shale with siltstone interbeds, and a

fine-grained greywacke. The base of the Martinsburg is calcareous shale as the lithology

transitions to a sequence of Ordovician carbonate formations (Becher and Root, 1981).

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Figure 2: Stratigraphic column representing the geology of the Cumberland Valley (from Becher and Root, 1981).

The geologic formations of the study area are discussed youngest to oldest,

consistent with the geologic law of superposition. The Chambersburg Formation is the

youngest of the Ordovician Carbonates within the study area. The thin-bedded

limestone weathers to cobbles and contains argillaceous partings and bentonite beds in

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the upper portion (Becher and Root, 1981). These bentonite beds are the last clay

deposits observed in the study area until reaching the Lower Cambrian Waynesboro

Formation.

The Saint Paul Group is a thick-bedded fossiliferous limestone with dolostone

interbeds and chert nodules (Becher and Root, 1981). The lithology transitions to

dolostone in the Pinesburg Station Formation. Quartz rosettes are present in the lower

portion of the Pinesburg Station Formation. The quartz rosettes continue into the

underlying Rockdale Run Formation, which is a fine-grained skeletal limestone (Becher

and Root, 1981). The Stonehenge, Stoufferstown, Shady Grove, and Zullinger

Formations are all predominately limestone formations.

The Elbrook Formation indicates a change in the depositional environment with

a change in the dominant lithology. The Elbrook Formation consists of a combination

of calcareous shale and argillaceous limestone (Becher and Root, 1981). The

Waynesboro Formation contains quartzite beds with worm burrows and ripple marks

(Becher and Root, 1981). The oldest formation in the study area is the Tomstown

Formation, a mottled dolostone with calcareous shale and limestone near the base

(Becher and Root, 1981).

METHODS

Specific conductivity (SpC) was measured in the field using a YSI ProPlus field

meter. Prior to data collection the meter was calibrated using a Ricca 250 microsiemens

per centimeter (µs/cm) specific conductivity standard. Due to freezing temperatures,

data collection sites were chosen from creeks fed by carbonate springs in the

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Cumberland Valley and included: Middle Spring Creek, Dykeman Spring Creek, and

Green Spring Creek. Each field measurement of SpC was multiplied by 0.48 to obtain

the hardness in mg/L of CaCO3 (Becher and Root, 1981). A water sample was collected

while measuring specific conductance in the field to perform a titration. The titration

was performed to confirm the results of the calculation applied to the field data from

Becher and Root (1981). The results of both tests were compared in table 2.

The water samples were titrated using 0.16 N sulfuric acid (H2SO4), consistent

with USGS protocol, to determine the amount of CaCO3. A Hach digital titrator was

used to add the H2SO4 with 800 drops of acid equaling 1 mg/L. The current USGS

standards require that water samples that are not filtered are titrated for acid

neutralizing capacity (ANC) instead of alkalinity. The results of the titrations were

entered into the U.S. Geological Survey’s online alkalinity calculator (U.S. Geological

Survey, 2013) using the inflection point method to calculate ANC and determine the

amount of CaCO3 present in each sample. The inflection point is set at a pH of 4.5 and

the results report the amount of calcium carbonate (CaCO3), bicarbonate (HCO3),

carbonate (CO3), and hydroxide (OH).

Cumberland and Franklin County Data

The existing water chemistry data from the Cumberland Valley was plotted in

Excel using a scatterplot to show the relationship between hardness and specific

conductance. Three outliers were investigated and determined to be due to data entry

errors after reviewing the published data (from Becher and Root, 1981; Becher and

Taylor, 1982). The errors were corrected and the correct data points added to the

scatterplot. Three existing outliers showing unique trends were explored. A trendline

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with a Y intercept of 0,0 was added to the scatterplot to determine the slope of the line.

The slope value was compared to the 0.48 multiplier (from Becher and Root, 1981).

Big Spring Data

Big Spring Creek originates from a limestone spring in the Shadygrove Formation

one mile north of Stoughstown, Pennsylvania and flows northeast to its confluence with

the Conodoguinet Creek near the borough of Newville, Pennsylvania (Greene, 2002).

Chemical and bacterial analyses of Big Spring Creek (Sp-22) were conducted in 1971 and

1974 and the results published in Becher and Root (1981). Pennsylvania State

University (Penn State) collected Ca, Mg, and SpC data for 13 wells in 2006. Dr. Feeney

provided these data for analysis in this study.

Hardness (CaCO3) for the Big Spring data was calculated using Equation 2. The

sum of the calcium (Ca in mg/L) divided by its atomic mass (40.08) and the magnesium

(Mg in mg/L) divided by its atomic mass (24.305) was multiplied by the molecular

weight of CaCO3 (100.09) to determine hardness. This calculation was performed in

Excel and the corresponding data points were added to the scatter plot created to show

the relationship between hardness and specific conductance in the Cumberland Valley.

305.24

)/(

08.40

)/(09.1003

LmgMgLmgCaCaCOasHardness

Equation 2: An equation used to calculate total hardness of water (Casiday and Frey, 1998).

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RESULTS

Field Sites

The sampling location at Green Spring Creek was on Hwy 641 in Newville,

Pennsylvania. The measurement was taken upstream of the bridge at the Green Spring

Brethren in Christ Church. The stream bed was predominately clay and silt. Algae and

primary producers were common in the water indicating a high nutrient content at this

location. The field measurement of SpC at this location was recorded as 702.3 µs/cm

(table 2). The SpC value was multiplied by 0.48 to obtain a hardness of 337.10 mg/L as

CaCO3. The ANC for this site reported a value of 344.2 mg/L as CaCO3.

Dykeman Spring was measured near the pressure transducer previously installed

by Shippensburg University, downstream from the culvert on Penn Street. The stream

bed at this location was predominately a pebble to cobble grain size with some sand and

silt. A SpC value of 373.3 µs/cm was measured in the field at this location. The

measured SpC was multiplied by 0.48, and the value of 179.18 mg/L CaCO3 was

compared to the ANC value of 179.4 mg/L as CaCO3 determined using the titration

results (table 2).

The Middle Spring sampling site was located on Stonewall Road, next to the

Cumberland Franklin Joint Municipal Authority pumping station #10 in Shippensburg,

and downstream from the University. The stream bed consisted of predominately clay

and silt sediments at this location. The field SpC reading was 645.3 µs/cm, resulting in

a calcium carbonate value of 309.74 after performing the calculation (645.3*0.48). The

ANC value of CaCO3 for this sample was reported as 224.6 mg/L (table 2).

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Table 2: Results of the analysis of three locations chosen within the Cumberland Valley.

Location SpC (µs/cm) Hardness

Calculated CaCO3

ANC Titration

Calculated CaCO3

Dykeman Spring 373.3 179.18 179.4

Green Spring 702.3 337.10 344.2

Middle Spring 645.3 309.74 224.6

Conductance and Hardness

The hardness and conductivity values have been plotted using an Excel

scatterplot in figure 3 to represent the overall trend of groundwater in the Cumberland

Valley. The Big Spring data calculated using equation 2 is consistent with the data

plotted using equation 1. All three field measurements are plotting near the trendline.

Three outliers were identified as having very high SpC and hardness values. Further

analysis of the data for these three outliers indicated that the Cl and Na levels are high at

all three locations (Becher and Root, 1981; Becher and Taylor, 1982).

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Figure 3: The water chemistry data from the published dataset shows the overall trend of hardness of groundwater in the Cumberland Valley (blue diamonds). The Big Spring data provided by Penn State is displayed in open red squares and the field sites are represented in triangles. The published datapoints that correspond to the Big Spring dataset and the field samples are represented in diamonds of the same color (see legend) (Becher and Root, 1981; Becher and Taylor, 1982).

DISCUSSION

The water hardness values in the published data for both Cumberland and

Franklin counties were calculated using the SpC value and the 0.48 multiplier

discovered by Becher and Root (1981). The y=mx+b equation generated in Excel after

the data were plotted was used to determine the validity of the 0.48 multiplier (from

Becher and Root, 1981). The slope of the trendline (m=0.4452) is within a reasonable

range to the 0.48 value; therefore, the use of Becher and Root’s (1981) 0.48 multiplier is

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accepted as valid unless there are elevated concentrations of anions such as chloride (Cl)

or sodium (Na) increasing the SpC. The outliers in the data in figure 3 are all due to

elevated Cl and Na.

The field sites were selected and SpC was measured. The measured value was

used to calculate the hardness as CaCO3 using the 0.48 multiplier discussed in Becher

and Root (1981). Titrations were performed in an attempt to confirm the validity of

using the multiplier for the three field locations. The results of the calculations are

compared to the results of the titrations in table 2. The ANC and calculated values for

CaCO3 in Dykeman Spring and Green Spring results were within an acceptable range.

However, the discrepancy in the Middle Spring results between the ANC value of CaCO3

and the calculated CaCO3 indicates that other ions are present in the sample. The

location is likely impacted by road salt from the Borough of Shippensburg and

Shippensburg University which are upstream of the sampling location. Further analysis

would likely indicate elevated chloride (Cl) or sodium (Na) in Middle Spring, similar to

the outliers in the published data. Increases of SpC do not guarantee an increase of

hardness of water.

The three field sites were plotted with the rest of the data from the Cumberland

Valley in figure 3 and were consistent with the trend, as were the Big Spring data. Many

of the data points fell on, or near, the trendline and are showing a high positive

correlation between SpC and hardness. The Green Spring Creek and Middle Spring

Creek points are located in the high conductance range of the scatterplot.

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Outliers

Outliers were identified after all of the data was plotted in the Excel scatterplot.

After reviewing the published data, three of the identified outliers were determined to be

data entry errors. These entries were corrected and the points plotted on the trendline

of the Cumberland Valley data. Two of those data points were apparent because it is

highly unlikely that SpC in a carbonate terrain would be less than 10 µs/cm. The

corresponding points were no longer outliers, but consistent with the observed trend.

Three outliers (figure 3) were investigated and identified as Cu-327, Fr-389, and

Fr-499. The Cumberland County well identified as Cu-327 is located in the Rockdale

Run Formation. The high conductance and hardness at this sampling location could be

due to the dissolution of the pure limestone by the large volume of water flowing

through this formation. Pure limestone has a high Ca content which is easily dissolved

by the flowing water. The Rockdale Run Formation contains karst features and the

limestone is very fine-grained with detrital and skeletal remains. Wells located in the

Rockdale Run Formation report a median specific capacity of 12 gal/min/ft and

sustained yields that reach 600 gal/min, with a median of 405 gal/min (Becher and

Root, 1981). It is likely that subsurface CO2 levels are high due to the karst features,

allowing more mineral dissolution to occur even though the residence time of the water

is short.

Well Fr-389 is located in Franklin County and has the largest conductance and

hardness values in the Franklin County dataset. Located in the Elbrook Formation, the

formation is Cambrian aged with a lithology ranging from limestone to calcareous shale

sandstone. The median specific capacity of the Elbrook is 2.0 gal/min/ft; this value is

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only surpassed by the Stonehenge Formation in Franklin County. The maximum

reported well yield is 250 gal/min (Becher and Taylor, 1982). The well record for Fr-

389 indicates this well is 90 ft deep and the water bearing zone is in the limestone.

The third outlier is Franklin County well Fr-499, located in the Martinsburg

Formation. The Martinsburg Formation has a median specific capacity of 0.80

gal/min/ft and a median reported well yield of 20 gal/min (Becher and Taylor, 1982).

Specific Capacity and Sustained Yield

The specific capacity of a well is useful in estimating the sustained yield, which

indicates the amount of water flowing through the aquifer. Specific conductance (SpC)

will typically increase as the flowpath of the water increases allowing the water to

dissolve more minerals along the way (Becher and Taylor, 1982). Increased residence

time of the water will also increase the SpC because more mineral dissolution occurs.

The outliers investigated had increased specific capacities and sustained yields, which is

inconsistent with the increased SpC measurements. Further investigation indicated

elevated Cl and Na concentrations elevated the SpC in those wells.

CONCLUSIONS

This study supports the relationship between SpC and hardness reported by

Becher and Root (1981). The 0.48 multiplier applied to SpC values is consistent unless

the water sample has been impacted by an outside source of dissolved ions, such as road

salt. The three creeks sampled during this study maintained the relationship stated by

Becher and Root (1981), also demonstrating the potential for miscalculation in a sample

with high SpC, such as Middle Spring Creek.

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Total hardness was calculated for the Big Spring dataset and added to the

scatterplot. The Big Spring data, collected by Penn State (2006) and provided by Dr.

Feeney, plotted with the trend of the published data and field samples, further

emphasizing the relationship between SpC and hardness.

Three outliers were due to data entry errors. After correcting the data using the

published data, the data plotted consistently with the observed trend. Three additional

outliers were explored and determined to have elevated Cl and Na concentrations. The

increase of SpC due to the Cl and Na made the relationship between SpC and hardness

invalid for those three wells.

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Becher, A., Taylor, L. 1982. Groundwater Resources in the Cumberland and Contiguous

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