Chapter 6 – Groundwater Quality

Release Date: December 2012

As described in Chapter 5 groundwater originates when surface water percolates through soil and rock and is stored beneath the earth’s surface in aquifers. In the Credit River Watershed, groundwater is the primary water source for approximately 116,000 people (CTC 2012). Groundwater is also interconnected with surface water (e.g. streams, rivers and wetlands) and therefore the quality of groundwater has an influence on surface water quality and the health of aquatic ecosystems. It is therefore important to prevent groundwater contamination because once this resource becomes polluted it is very difficult and costly to clean. Monitoring groundwater quality helps identify potential future problems as part of effective groundwater management.

Groundwater is vulnerable to contamination from human activities. For example, leaks from septic, fuel storage tanks or landfills, as well as the applications of road salt, pesticides or fertilizer all contaminate groundwater. The faster a contaminant migrates through the ground to the aquifer without being diluted or rendered less harmful, the more vulnerable the groundwater is to contamination. This vulnerability of groundwater is determined by factors such as the depth and thickness of the aquifer, the types of soils overlying the aquifer, and the rate at which water travels through the ground to the aquifer. The vulnerability and potential threats to groundwater around municipal wells in the Credit River Watershed is summarized in the Source Protection Report (CTC 2012).

Groundwater Quality Monitoring

CVC is collecting groundwater quality data from 14 wells as part of the Provincial Groundwater Monitoring Network (PGMN; Figure 2). The sampling methodology follows the PGMN Long-term Groundwater Quality Sampling Program (MOE 2009). Groundwater quality in the Credit River Watershed is ranked using two parameters, chloride and nitrogen (nitrate+nitrite). Chloride is naturally occurring in water but human activities such as road salting or leaking septic systems can increase chloride concentrations. At high concentrations chloride can produce a salty taste (MOE 2006) and impact the ability of organisms to effectively osmoregulate (CCME 2011). Nitrogen, is also naturally occurring (particularly in ground water), however, human activities such as fertilizer application and loading from sewage treatment plants can artificially elevate nitrogen concentrations (CCME 2012). At high nitrogen concentrations the ability of red blood cells to carry oxygen is reduced posing a potential risk to both aquatic organisms (CCME 2012) and humans (particularly babies and small children; MOE 2006).

Figure 2: Groundwater quality monitoring stations in the Credit River Watershed and their status (2011)

As part of the Integrated Watershed Monitoring Program (IWMP; 2002-2011) groundwater quality was assessed in comparison to Conservation Ontario’s groundwater quality index (Table 1). This index is based on the Ontario Drinking Water Standards Guidelines (ODWS; MOE 2006). ODWS has defined an aesthetic objective for chloride of 250 mg/L and a maximum acceptable concentration for nitrogen of 10 mg/L. Conservation Ontario uses these ODWS drinking water guidelines to define the lowest score on the groundwater quality index (a score of 1). In addition to Conservation Ontario’s index, groundwater quality in this report is also compared to the Canadian Water Quality Guidelines for the Protection of Aquatic Life (CWQG). Although these guidelines were established for surface water the tight connections between groundwater and surface water in the Credit River Watershed makes these values of interest when discussing groundwater quality. The CWQG for long-term exposure is established as 120 mg/L for chloride (CCME 2011) and 3.0 mg/L for nitrogen (nitrate; CCME 2012).

Table 1. Conservation Ontario index

Chloride (mg/L)

Nitrogen (mg/L)

Score


0 – 62.5

0 – 2.5

5

62.6 – 125.0

2.6 – 5.0

4

125.1 – 187.5

5.1 – 7.5

3

187.6 – 250.0

7.6 – 10.0

2

> 250.0

> 10.0

1

Groundwater Quality Status (2011)

Overall, groundwater quality at most wells in the Credit River Watershed is excellent to good (Figure 3). In 2011, approximately 80% (11 of 14 wells) of sampling sites received a score of 4 or 5 for chloride and nitrogen concentrations. Some sites, however, received the lowest score (1) for chloride (2 wells) and nitrogen (1 well) indicating local groundwater quality concerns. It is important to note that because there is a single sampling well in the Lower Watershed it is difficult to properly assess groundwater quality in that region.

Figure 3: Distribution of groundwater quality scores by watershed physiographic zone for the 14 PGMN wells in the Credit River Watershed.

Groundwater Quality Trends

From 2002-2011, although groundwater quality was variable, most of the wells either did not change in their Conservation Ontario groundwater quality index score or moved only a single position.

In the Upper Watershed increased chloride concentration was the most prominent trend. Statistically significant increases in chloride concentrations were observed in five of the six PGMN wells, presumably in response to winter road salt application (See Table A1and A2 for results of statistical trend analysis). The one exception to this increasing chloride was the Hillsburgh Shallow well, which had a statistically significant decline in chloride concentrations. It is important to note that although chloride concentrations have been increasing in the Upper Watershed, they remained low and below ODWS and CWQG guidelines (Figure 4a).

Nitrogen concentrations in the Upper Watershed, although variable, showed no statistically significant trends over time (Figure 4b). The Hillsburgh Shallow and Deep wells, however, had nitrogen concentrations consistently above the CWQG long-term guideline for the protection of aquatic life and therefore groundwater quality in these wells will continue to be carefully monitored for potential impacts on aquatic ecosystems.

Figure 4: a) Chloride and b) nitrogen concentration trends in the Upper Watershed.

In the Middle Watershed, the only statistically significant trend in chloride concentration was a decline in chloride at the Georgetown Deep well. This well had an average chloride concentration of approximately 670 mg/L over the seven year monitoring record, well above the CWQG long-term guideline of 120 mg/L and the ODWS Aesthetic Objective Level of 250 mg/L (Figure 5a). Although chloride concentrations in the Georgetown Shallow well remained high, chloride concentrations in this well have been improving from a high of approximately 1000 mg/L in 2005 to 400 mg/L in 2011. The other striking record of chloride comes from the Warwick Shallow well which also consistently had chloride concentrations well above both the CWQG and ODWS guidelines. These results suggest local groundwater quality issues that will continue to be closely monitored by CVC for possible impacts on aquatic ecosystems.

Nitrogen concentrations in the Middle Watershed have been significantly increasing in both the Robert Baker Shallow and Deep monitoring wells and significantly decreasing in the Acton Deep well (Figure 5b). Despite the increasing trend in the Robert Baker wells, nitrogen concentrations remained very low (average approximately 0.035mg/L) placing it well within the top category for groundwater quality. For the Acton Deep monitoring well, nitrogen concentrations have been significantly declining over the last seven years and since 2006 have received the best possible score on the groundwater quality index, indicating improving conditions at this site. The decline in nitrogen concentration in the Acton Deep well, however, is likely the result of a decrease in fertilizer application as this well is located in a former agricultural field that is now part of a planned quarry.

In the Lower Watershed, monitoring records are not of sufficient length to provide groundwater quality trends at this time. Measurements of chloride concentrations indicate that the single well in the Lower Watershed are below both the CWQG and ODWS guidelines (Figure 6). In contrast nitrogen concentrations are well above CWQG and ODWS guidelines likely as a result of fertilizer application as this well sits in an agricultural field. There is however preliminary evidence that nitrogen concentrations are beginning to decline at this location as the field is converted into residential homes.

Conclusions

Overall, groundwater quality is excellent to good with most monitoring wells scoring in the top categories on Conservation Ontario’s groundwater quality index. Although six wells had an increasing trend in chloride or nitrogen concentrations, their concentrations remained low and were within (or very near) the best possible score on the groundwater quality index. Furthermore, for the three wells that received a poor score on the groundwater quality index, two recorded trends of improving conditions while the third showed no significant trend. Therefore, although groundwater quality remains rated as poor in these three sites, the trend suggests that groundwater quality in at least two of them is improving. As CVC continues to monitor groundwater we will be better positioned to provide informed analysis of changes to the quality of this valuable natural resource.

 

The next chapter will look at an IWMP component closely tied to climate: streamflow. Chapter 7 will describe seasonal patterns in streamflow and also present short and long-term trends in streamflow across the watershed.

Did you know?

Environment Canada has established 10 streamflow monitoring stations in the Credit River Watershed. One of these stations dates back to World War I and is almost 100 years old.

 

References

CCME. 2011. Canadian Water Quality Guidelines : Chloride Ion. Scientific Criteria Document. Canadian Council of Ministers of the Environment, Winnipeg.

CCME. 2012. Canadian Water Quality Guidelines : Nitrate Ion. Scientific Criteria Document. Canadian Council of Ministers of the Environment, Winnipeg.

CTC Source Protection Region. 2012. Proposed Source Protection Plan: CTC Source Protection Region.

MOE (Ministry of Environment). 2006. Technical Support Document for Ontario Drinking Water Standards, Objectives and Guidelines.

MOE (Ministry of Environment). 2009. Provincial Groundwater Monitoring Network Sampling Protocol: A Guide to the Collection And Submission of Groundwater Samples for Analysis.

 

Table A1: Results from linear regression analysis of groundwater nitrogen concentration (mg/L) versus monitoring year.

Monitoring well

Sample size (n)

Coefficient of determination (r2)

Slope

p value

Upper Watershed

Orangeville Deep

5

0.76

-0.01

0.053

Orangeville Shallow

4

0

0

Erin

8

0.17

0.14

0.31

Caledon

6

0.19

0.03

0.39

Hillsburgh Deep

7

0.02

0.03

0.77

Hillsburgh Shallow

7

0.13

0.10

0.43

Middle Watershed

Georgetown Deep

6

0.05

0.04

0.64

Georgetown Shallow

6

0.02

-0.01

0.78

Warwick Deep

6

0.23

-0.09

0.16

Warwick Shallow

9

0.59

0.29

0.026

Robert Baker Deep

4

0.76

0.01

0.01

Robert Baker Shallow

5

0.70

0.01

0.02

Acton

9

0.78

-0.39

0.002

Lower Watershed

Huttonville

2

Table A2: Results from linear regression analysis of groundwater chloride concentration (mg/L) versus monitoring year.

Monitoring well

Sample size (n)

Coefficient of determination (r2)

Slope

p value

Upper Watershed

Orangeville Deep

6

0.72

0.70

0.03

Orangeville Shallow

6

0.84

1.00

0.01

Erin

9

0.47

3.47

0.04

Caledon

7

0.82

1.54

0.005

Hillsburgh Deep

8

0.27

-0.47

0.18

Hillsburgh Shallow

8

0.75

-2.49

0.006

Middle Watershed

Georgetown Deep

7

0.44

-1.40

0.11

Georgetown Shallow

7

0.65

-72.72

0.03

Warwick Deep

9

 

 

 

Warwick Shallow

9

 

 

 

Robert Baker Deep

8

0.42

-0.10

0.08

Robert Baker Shallow

8

0.08

-0.07

0.51

Acton

9

0.03

-0.12

0.66

Lower Watershed

Huttonville

2

Data and information released from Credit Valley Conservation (CVC) are provided on an 'AS IS' basis, without warranty of any kind, including without limitation the warranties of merchantability, fitness for a particular purpose and non-infringement.

Availability of this data and information does not constitute scientific publication. Data and/or information may contain errors or be incomplete. CVC and its employees make no representation or warranty, express or implied, including without limitation any warranties of merchantability or fitness for a particular purpose or warranties as to the identity or ownership of data or information, the quality, accuracy or completeness of data or information, or that the use of such data or information will not infringe any patent, intellectual property or proprietary rights of any party. CVC shall not be liable for any claim for any loss, harm, illness or other damage or injury arising from access to or use of data or information, including without limitation any direct, indirect, incidental, exemplary, special or consequential damages, even if advised of the possibility of such damages.

In accordance with scientific standards, appropriate acknowledgment of CVC should be made in any publications or other disclosures concerning data or information made available by CVC.
DATA DISCLAIMER