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Elevated arsenic in private wells of Cerro Gordo County, Iowa: causes and policy changes.


One of the most challenging environmental health problems today, known to affect millions of people worldwide, is arsenic-contaminated drinking water (Amini et al., 2008; Bhattacharya et al., 2007; Nordstrom, 2002; Smith, Lopipero, Bates, & Steinmaus, 2002). The World Health Organization (WHO, 2010, 2016) guideline for arsenic in drinking water is 10 micrograms per liter (pg/L). The U.S. Environmental Protection Agency has also set the maximum contaminant level (MCL) for arsenic at 10 pg/L (U.S. EPA, 2002). U.S. EPA regulates public water systems, but does not have the authority to regulate drinking water in private wells (U.S. EPA, 2016). Thus, many private wells are not tested for arsenic.

A complete literature review on arsenic contamination in groundwater and the resulting health effects is beyond the scope of this article. There are, however, some excellent review summaries on arsenic contamination in groundwater (Ahuja, 2008; Khan, Sakauchi, Sonoda, Washio, & Mori, 2003; Mukherjee et al., 2006; Ravenscroft, Brammer, & Richards, 2009; Welch, Lico, & Hughes, 1988). In addition, organic arsenic is not readily eliminated by the body, compounding chronic negative health effects (Bates, Smith, & Hopenhayn-Rich, 1992; Flora, 2015). In particular, chronic arsenic exposure can result in skin lesions, keratosis, peripheral neuropathy, gastrointestinal symptoms, renal system effects, high blood pressure, reproductive problems, cardiovascular disease, and cancer (Bhattacharya et al., 2007; Chen et al., 2009; Kapaj, Peterson, Liber, & Bhattacharya, 2006; Navas-Acien et al., 2005; Navas-Acien, Silbergeld, Pastor-Barriuso, & Guallar, 2008; Ng, Wang, & Shraim, 2003; Nordstrom, 2002; Smedley & Kinniburgh, 2002; WHO, 2010, 2016).

Arsenic in drinking water was initially discovered in Cerro Gordo County in the 1990s, with the extent of the problem becoming better known in recent years. Rural populations in the Midwest might be at higher risk, as they often tend to be less transient and families drink water from the same well source for many years. Arsenic in Iowa wells has been relatively unstudied (Schnoebelen & Walsh, 2014a, 2014b). The Iowa Statewide Rural Well Water Survey Phase 2 in 2005, however, showed that arsenic was present in 47% of the wells tested, with elevated arsenic levels found in 33 counties, including Cerro Gordo County (Center for Health Effects of Environmental Contamination, 2017).

Most private wells in the study area are open at depths between 100-400 feet and utilize the 1) Devonian Lime Creek Formation (Lime Creek Aquifer), the upper aquifer or 2) the Devonian Cedar Valley Group (Cedar Valley Aquifer), the lower aquifer. The limestone and dolostone formations of the aquifers are accompanied by minor shale deposits and pyrite (Iowa Department of Natural Resources [IDNR], 2013; Prior, Boekhoff, Howes, Libra, & VanDorpe, 2003). The Cedar Valley Aquifer exceeds 350 feet in thickness in places, is deeper, contains less shale, and yields more water than the Lime Creek Aquifer.

This 5-year study was funded through the Environmental Health Specialist Network (EHS-Net) Water Program at the Centers for Disease Control and Prevention (CDC) beginning in 2010 (CDC, 2014a, 2014b). The partners involved included the University of Iowa, Iowa Department of Natural Resources (IDNR), Iowa State Hygienic Laboratory (SHL), and Shawver Well Company. The diverse team had experts in public health and communication, analytical chemistry, geochemical modeling, geology, and well drilling.

The study objectives for Cerro Gordo County were to identify the source, mobilization, and distribution of arsenic in groundwater. In addition, the study team embarked on a strong education and outreach campaign to educate and inform private wells owners throughout the study.


Well Selection

Potential wells were selected using data from the IDNR GeoSAM database (IDNR, 2017). Criteria for selection included the most complete information for well identification number, location, depth, drilling date, owner, elevation, casing depth, casing into bedrock depth, bedrock elevation, total depth, static water level, pumped water level, well yield, drilling log, aquifer name, rock chip samples, and spatial distribution.

Several recruitment methods to engage participants in the study were employed including presenting at town hall meetings, using social media, and issuing a press release. Participant invitation packets were sent to 108 well owners who met the criteria. The response rate was approximately 60% positive to participate (65 positive responses initially), with three additional wells added throughout the study for a total of 68 wells. Figure 1 shows the final arsenic study well sampling sites.

Water Quality Sampling and Laboratory Analysis

Study protocol dictated sampling twice per year for any temporal variation in arsenic (wet period: May-September and dry period: October-April) over the 3-year period. Water quality collection methods followed those used by federal agencies (U.S. EPA Region 1, 2010; U.S. Geological Survey, 2006). The chemical analysis and arsenic speciation work was done at SHL facilities (standard U.S. EPA methods 200.7 and 200.8), in Ankeny, Iowa (Iowa State Hygienic Laboratory [SHL], 2016a). SHL is accredited by the National Environmental Laboratory Accreditation Program in conjunction with numerous other certifications (SHL, 2016b). The first column of Table 1 lists field parameters and laboratory analysis for each sample.

Sampled wells were pumped and monitored for approximately 25-30 minutes prior to sampling from a faucet or valve before any household water treatment equipment. New tubing was used for each well sampled to eliminate the chance of "carry over" of chemical species between sample sites. The sampling process involved the routine measurements of pH, temperature, specific conductance, and dissolved oxygen.

Approximately 9% of the total 393 samples collected (35 samples) were field replicate samples collected sequentially immediately after the regular environmental sample for quality control. Samples were shipped on ice by overnight express to SHL for analysis.

Data Analysis

The R statistical package was used for statistical analysis of the water quality results (R-Statistical Computing, version 3.2.1, 2015). Statistics included minimum, 25th percentile, median, mean, 75th percentile, and maximum, in addition to correlation and hypothesis testing. The pH-REdox-EQuilibrium geochemical program version 3.0 (PHREEQC) was used to simulate potential chemical reactions speciation, and the calculation of saturation indices (SI) for numerous mineral species (Parkhurst & Appelo, 2013).


Arsenic Detections

Arsenic was detected in wells throughout the county (Figure 1). A total of 393 water samples were collected from 68 wells during 2011-2013. In quality control samples, the environmental and replicate samples matched closely as 32 of the 35 replicate samples were the same as the environmental samples with three replicate samples varying from the environmental sample by only 0.001 pg/L. These data indicate consistent repeatability of results. Only the environmental samples (i.e., no replicates) were used in computing final summary statistical water quality results. Additionally, eight water quality samples were not included in the final statistical analysis as these were missing some general chemical data (trace metal data, but not arsenic data). This exclusion left 350 total water quality samples for the final statistical water quality analysis and 358 samples for arsenic analysis.

Water Quality

Statistics (minimum, 25th percentile, median, mean, 75th percentile, and maximum) were calculated for 350 water quality samples (Table 1). In general, the water chemistry is dominated by a calcium-bicarbonate rich groundwater (calcium mean = 72.6 mg/L; bicarbonate mean = 341.8 mg/L), together with sodium (mean = 20.83 mg/L) and sulfate (mean = 13.47 mg/L). These results are typical of limestone and dolostone aquifers in Iowa where the bedrock groundwater is dominated by these ions (confirmed by total alkalinities that ranged from 170-460 mg/L and total dissolved solids ranged from 210-730 mg/L).

We found that 68 samples of the 358 total had detections of arsenic. Table 2 shows there were 31 samples with detections of arsenic at or above the MCL of 10 [micro]g/L, 75 samples had arsenic detected between 1-10 [micro]g/L, and 252 samples with arsenic below the detection level (<1 [micro]g/L). The highest detected arsenic concentration in the study was 110 [micro]g/L. There were 79 rock chip samples analyzed for arsenic in the Cedar Valley and Lime Creek aquifers.

Box plots of arsenic concentrations by aquifer formation (Figure 2) and those of rock chip samples from the Cedar Valley and Lime Creek aquifers (Figure 3) are provided. The results of the seasonal wet (May-September) and dry (October-April) sampling periods are shown in Table 2.


Understanding the source and mobilization of arsenic from rock into water was important for making future public health decisions in the county. Typically, arsenic can change oxidation state and As(III) is more toxic and mobile than As(V) (Welch, Westjohn, Helsel, & Wanty, 2000; WHO, 2011). During sample analysis, if total arsenic was detected at or above 5 [micro]g/L, arsenic speciation was performed. On average there was a 50% split of arsenic as As(III) and As(V) when arsenic speciation was done for the total sample set. Variation of the speciation, however, for individual water samples was more pronounced with As(III) composing 6-100% of the total arsenic in some samples and As(V) composing 4-92% of total arsenic in other samples. This finding indicated that if homeowners detect arsenic they should run the speciation to see if they have the more toxic arsenic, As(III).

The Lime Creek Aquifer has more shale and pyrite than the Cedar Valley Aquifer. Indeed, pyrite (FeS2) is one of the most common iron sulfide minerals and has been shown to incorporate large (up to 10.0 wt %) amounts of arsenic in its structure (Abraitis, Pattrick, & Vaughan, 2004). Pyrite and other sulfide minerals are often found in shales and carbonate bedrock as small (2-20 pm) framboids (Schieber, 2011; Smedley & Kinniburgh, 2002, 2005). Arsenic concentration in rock and well water samples were higher for the Lime Creek Aquifer than for the Cedar Valley Aquifer (Figures 2 and 3). Rock chip samples showed arsenic concentration in the Lime Creek bedrock at a mean of 11.4 mg/g compared with the Cedar Valley bedrock of 1.2 mg/g. The shales and pyrite were identified as a source of the arsenic in the Lime Creek Aquifer.

Initially, the arsenic detections (arsenic [greater than or equal to] 10 [micro]g/L and 1 [less than or equal to] arsenic <10 [micro]g/L) were statistically compared between the water quality parameters listed in Table 1 to determine if there was any significance in a particular water quality parameter correlated with detectable arsenic concentrations using Pearson's product-moment correlation r, where r is a measure of strength of a linear association between two variables. Hypothesis testing using the p-value was completed as well. Potential correlations (or alternatively no correlations) are typically interpreted as numbers less than 0.05 or greater than 0.05, respectively. The hypothesis testing was completed for two sets: 1) arsenic equal to 10 [micro]g/L or greater and 2) arsenic equal to 1.0 [micro]g/L or greater but less than 10 [micro]g/L. In general, the p-value hypothesis testing did not show any strong correlation with other chemical parameters that might have been used as a surrogate for arsenic in the future. Selected results are shown in Table 3. The p-value hypothesis tests, however, did indicate that shallower bedrock depth and increased change in water level did show statistical significance for arsenic [greater than or equal to] 10 [micro]g/L (p = .0015, r = 0.707 and p = .0009, r = 0.658, respectively) (Table 3). Similarly, for arsenic greater than 1.0 pg/L but less than 10 [micro]g/L, increased dissolved oxygen and shallower bedrock depth were correlated with increased arsenic. Shallow wells in the Lime Creek Aquifer with oxic water were at risk for arsenic contamination (Table 3).

SI and Potential Geochemical Reactions

The SI calculations by PHREEQC also indicated a dominance of more oxic iron sulfide minerals formation (i.e., hematite, magnetite, and goethite) expected in the mobilization of arsenic under more oxygen-rich conditions. These data were additionally confirmed by Eh values that were more oxic (less negative; mean = -76 mV, Table 1) compared with reducing conditions (i.e., Eh = -200 to -400 mV). Finally, iron and manganese were at relatively low concentrations in the groundwater samples (iron mean = 0.750 mg/L and manganese mean = 0.030 mg/L), again supporting more oxic water. Iron and manganese concentrations would typically be over 1.0 mg/L if the conditions were more reducing (Chapelle & Lovley, 1992). These geochemical data support the hypothesis of oxic conditions where arsenic could mobilize from the rock into the water. The PHREEQC SI calculations illustrated groundwater was dominated by calcium, manganese, sodium, and bicarbonate, which corresponded to actual water sample data, confirming that PHREEQC was yielding accurate results.

Pyrite was present in the aquifer and is known to contain arsenic in the Lime Creek Aquifer (Figure 3); thus, the oxidation of pyrite is a potential pathway for arsenic into water. In the presence of aerated (oxygenic) water, dissolved Fe(II), pyrite, and other Fe(II) sulfides containing arsenic can readily oxidize to Fe(III) (Gleisner & Herbert, 2002), releasing As into the water. Thus, in carbonate-dominated groundwater (Table 1), Fe(III) can readily react with water, oxygen, and/or hydroxides to become saturated in the groundwater (i.e., yielding saturation indices >1) to form (oxy) (hydr)oxides (shown in the PHREEQC data runs). These Fe hydroxides often cause "iron staining" in pumping fixtures as further con firmed by homeowners in Cerro Gordo County with untreated well water.

Finally, when examining the hydrologic properties of the Lime Creek Aquifer from drilling logs, it was found that often the pumped water levels in the Lime Creek aquifer could drop several feet below the static water level. In some instances, we noted a 60-100 feet of drop (hydraulic head difference). This drop has the potential to introduce oxygen into the aquifer through the wellbore. The Lime Creek Aquifer does not have as large of specific capacity (pumping rate or well yield divided by drawdown) under pumping as the Cedar Valley Aquifer due to the shales in this aquifer. This limited capacity means the Lime Creek Aquifer will have larger drawdowns under pumping (introducing more oxygen to the water) than the Cedar Valley Aquifer.

Communicating Results to Residents

Project participants, private well users, and the general public received regular project updates through multiple methods. Forming these strong community-based research partnerships was critical for communicating health effects (Israel, Schulz, Parker, & Becker, 1998). Participants received copies of all analyses completed on their wells along with education. One-on-one mentoring and easy-to-understand videos helped communicate results across all educational levels. We created three videos, including one that featured an interview with a woman who had suffered health effects from consuming arsenic in her groundwater (http://

Health Policy Change

The results of our study were presented to the Cerro Gordo County Board of Supervisors in March 2015. Our study clearly showed that arsenic was a problem countywide, with the source and mobilization of arsenic in the Lime Creek Aquifer. The County Well Ordinance was rewritten based on the results of our study to include language that all new wells drilled must be cased through the upper Lime Creek formation (the aquifer with the greatest source of arsenic) and that all future wells would be tested for arsenic when put in service. County health officials now had the correct information to provide to residents and could provide possible treatment options if wells were affected. In addition, results from this study influenced a statewide revision of rules to allow for arsenic testing in the Grants to Counties bacteria and nitrate private well testing program. Nationally, the team was involved in a U.S. EPA workgroup that wrote a report to Congress regarding potential rule changes for small public water systems and arsenic contamination.


We met our objectives within this study on determining the distribution of groundwater arsenic concentrations, identification of arsenic sources, and establishment of best practices for future well construction to minimize risk for wells in Cerro Gordo County, Iowa. A critical part of the study involved educating private well owners on the risks of arsenic in groundwater and providing good communication by team members on research results. The arsenic source was naturally occurring sulfide minerals (pyrite) containing arsenic commonly associated with the Lime Creek Aquifer. Geochemical analysis suggests that arsenic is most likely mobilized through oxidizing conditions, particularly in shallow aquifers that are subject to larger water level changes during pumping. Reducing conditions, however, could also mobilize arsenic in deeper parts of the aquifer system in places. The seasonal and long-term variations in arsenic were minimal for individual wells. Future studies are warranted to expand and fill known geographical, environmental, and public health sampling gaps for arsenic in groundwater, geochemistry, and biomarkers. The results of the study support other arsenic issues in the Midwest with similarities to private wells and arsenic in groundwater (Erickson & Barnes, 2005; Minnesota Department of Health, 2001, 2008, 2015; Wisconsin Department of Natural Resources, 2016a, 2016b), but not previously identified in Iowa.

Douglas J. Schnoebelen, PhD

The University of Iowa

Sophia Walsh

Brian Hanft, MPA, REHS

Cerro Gordo County Department of Public Health

Oscar E. Hernandez-Murcia, PhD

The University of Iowa

Chad Fields, MS

Iowa Department of Natural Resources

Acknowledgements: The authors wish to acknowledge the following agencies and individuals who were vital in making this project a success: Cerro Gordo County Department of Public Health: Dan Ries, REHS, and Kara Vogelson; University of Iowa/IIHR-Hydroscience & Engineering; Iowa Department of Natural Resources: Paul Van Dorpe; Iowa State Hygienic Laboratory: Lorelei Kurimski, MS, Sherri Marine, Pam Mollenhauer, Don Simmons, PhD, Michael Wichman, PhD, and Brian Wels, PhD; Shawver Well Company, Inc.: Ryan Budke and Gary Shawver; Center for Health Effects and Environmental Contamination: Peter Weyer, PhD; Iowa Geological Survey: Robert Libra; and CDC's EHS-Net Water Program: Max Zarate-Bermudez, MSc, MPH, PhD, Daneen Farrow-Collier, MSPH, and Connie Thomas. The authors are deeply grateful to all these individuals who were instrumental in their time and support of the project from initial planning, sampling, conferences, and laboratory work.

Corresponding Author: Douglas J. Schnoebelen, Chief, South Texas Program Office, U.S. Geological Survey, 5563 De Zavala Road, San Antonio, TX 78249.



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Caption: FIGURE 1 Map of Cerro Gordo County, Iowa, With Well Sampling Locations and Areas Where Arsenic Was Detected in the Study

Caption: FIGURE 2 Arsenic Concentrations From Water Samples by Aquifer Type

Caption: FIGURE 3 Arsenic Concentration From Rock Chip Samples by Aquifer Type

Water Quality Characteristics and Field Parameters of 68 Wells (350
Samples) in Cerro Gordo County, Iowa (2011-2013)

Compound                          Units           Minimum

Iron                               mg/L            0.01
Magnesium                          mg/L            0.25
Manganese                          mg/L            0.01
Sodium                             mg/L             3.0
Zinc                               mg/L            0.01
Turbidity                          NTU              0.5
Nitrate-N                       mg/L as N          0.05
Sulfate                            mg/L             0.5
Ammonia-N                       mg/L as N          0.025
Bicarbonate                        mg/L             0.5
Carbonate                  mg/L as CaC[O.sub.3]     0.5
Chloride                           mg/L             0.5
Dissolved organic carbon           mg/L            0.25
Total alkalinity                   mg/L             170
Arsenic                            jg/L             0.5
Cadmium                            mg/L           0.0005
Calcium                            mg/L             0.5
Copper                             mg/L            0.005
Total dissolved solids             mg/L            210.0
Nickel                             mg/L            0.025
Arsenic (MI) *                     mg/L             0.5
Arsenic (V) *                      mg/L             0.5
Temperature                     [degrees]C          3.8
Specific conductance              jS/cm            317.3
pH                                  --              6.8
Eh                                  mV            -278.1
Dissolved oxygen                   mg/L              0

Compound                   1st Quartile   Median    Mean

Iron                           0.05       0.190     0.750
Magnesium                      27.0        32.0     30.76
Manganese                      0.01        0.01     0.030
Sodium                         12.0        18.0     20.83
Zinc                           0.01        0.01     0.027
Turbidity                      0.5         1.6      8.34
Nitrate-N                      0.5         0.5      0.557
Sulfate                        2.02        7.45     13.47
Ammonia-N                      0.26        0.46     0.631
Bicarbonate                    330        355.0     341.8
Carbonate                      0.5         0.5      3.42
Chloride                       1.3         2.7      5.93
Dissolved organic carbon       0.9         1.3      1.39
Total alkalinity               330        360.0     343.9
Arsenic                        0.5         0.5      0.516
Cadmium                       0.0005      0.0005   0.00051
Calcium                        66.0        72.0     72.57
Copper                        0.005       0.005    0.0068
Total dissolved solids        330.0       360.0     361.6
Nickel                        0.025       0.025     0.025
Arsenic (MI) *                 0.5         4.1      12.95
Arsenic (V) *                  1.95        3.9      12.68
Temperature                    11.8        13.3     13.61
Specific conductance          531.7       609.2     604.1
pH                            7.385        7.92     8.028
Eh                            -104.6      -73.7    -76.56
Dissolved oxygen               0.07        0.12    0.6371

Compound                   3rd Quartile   Maximum

Iron                           0.49        20.0
Magnesium                      36.0        47.0
Manganese                      0.02         0.4
Sodium                         23.0        170.0
Zinc                           0.03        0.19
Turbidity                      5.8         230.0
Nitrate-N                      0.5         15.0
Sulfate                        17.0        89.0
Ammonia-N                      0.85         2.9
Bicarbonate                   380.0        460.0
Carbonate                      0.5         370.0
Chloride                       6.5         90.0
Dissolved organic carbon       1.7          6.1
Total alkalinity              380.0        460.0
Arsenic                        1.0         110.0
Cadmium                       0.0005       0.004
Calcium                        77.0        150.0
Copper                        0.005        0.05
Total dissolved solids        390.0        730.0
Nickel                        0.025        0.025
Arsenic (MI) *                 12.0        91.0
Arsenic (V) *                  15.5        95.0
Temperature                   15.25        21.1
Specific conductance          669.0       1,059.0
pH                             8.46        11.78
Eh                            -48.9        728.4
Dissolved oxygen              0.305        12.57

* If total arsenic was detected at or above 5 [micro]g/L,
arsenic speciation was performed.


Total Arsenic Data for All Environmental Samples Collected and
Analyzed for the Study With Subsets of Wet Period (May-September)
and Dry Period (October-April) Samples

Arsenic Range           Total # of     Wet Period        Dry Period
                         Samples         Samples           Samples
                                     (May-September)   (October-April)

Arsenic [greater than       31             20                12
  or equal to] 10
1 [less than or             75             26                31
  equal to] Arsenic
  <10 [micro]g/L
Arsenic <1 [micro]g/L      252             137               132
Totals                     358             183               175


Hypothesis Testing Results for Arsenic Concentrations Versus
Selected Parameters

Sets                             Dissolved        Eh        Bedrock
                                   Oxygen                    Depth

Arsenic [greater than or equal    p = .245     p = .181    p = .0015#
to] 10 [micro]g/L                r = 0.228    r = 0.251    r = 0.707#
                                   n = 31       n = 31       n = 31
                                   NA = 3       NA = 1      NA = 14

1 [less than or equal to]        p = .0012#    p = .989    p = .044#
Arsenic <10 [micro]g/L           r = 0.374#   r = 0.0016   r = 0.236#
                                   n = 75       n = 75       n = 75
                                   NA = 0       NA = 0       NA = 2

Sets                              Change in     Nitrate     Sulfate
                                 Water Level

Arsenic [greater than or equal   p = .0009#    p = .114    p = .474
to] 10 [micro]g/L                r = 0.658#    r = 0.294   r = 0.133
                                   n = 22       n = 31      n = 31
                                   NA = 0       NA = 1      NA = 0

1 [less than or equal to]          p = .68     p = .791    p = .117
Arsenic <10 [micro]g/L            r = 0.163    r = 0.031   r = 0.182
                                   n = 73       n = 75      n = 75
                                   NA = 0       NA = 0      NA = 0

NA = not available.

Note. Bolded text indicates statistical significance.

Note: Statistical significance are indicated with #.
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Article Details
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Author:Schnoebelen, Douglas J.; Walsh, Sophia; Hernandez-Murcia, Oscar E.; Fields, Chad
Publication:Journal of Environmental Health
Article Type:Report
Date:May 1, 2017
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