Ground Water Rule Compliance 

Monitoring: A Quick Reference Guide

Overview of the Rule

Title Ground Water Rule (GWR) 71 FR 65574, November 8, 2006, Vol. 71, No. 216 

Correction 71 FR 67427, November 21, 2006, Vol. 71, No. 224

Purpose Reduce the risk of illness caused by microbial contamination in public ground 

water systems (GWSs).

General 

Description

The GWR establishes a risk-targeted approach to identify GWSs susceptible 

to fecal contamination and requires corrective action to correct significant 

deficiencies and address source water fecal contamination in all public GWSs. 

Utilities 

Covered

The GWR applies to all public water systems (PWSs) that use ground water, 

including consecutive systems, except that it does not apply to PWSs that 

combine all of their ground water with surface water or with ground water 

under the direct influence of surface water prior to treatment. 

Purpose of Compliance Monitoring

► Compliance monitoring ensures that GWSs that provide at least 4-log treatment of viruses 

using chemical disinfection, membrane filtration, or a State-approved alternative treatment 

technology are consistently and effectively achieving this level of treatment.

When is Compliance Monitoring Required?

► GWSs that provide at least 4-log treatment of viruses as a corrective action must conduct 

compliance monitoring.

► GWSs that provide at least 4-log treatment of viruses at or before the first customer 

using chemical disinfection, membrane filtration, or a State-approved alternative treatment 

technology and do not conduct GWR triggered source water monitoring must notify their 

State in writing that they provide treatment and begin compliance monitoring. 

► The compliance dates for systems that provide 4-log treatment in lieu of GWR triggered 

source water monitoring are as follows:

 GWSs with existing ground water sources must notify the State by December 

1, 2009, that they provide at least 4-log treatment of viruses and begin compliance 

monitoring.

 GWSs with new ground water sources placed into service after November 30, 2009, 

must notify the State that they provide at least 4-log treatment of viruses and begin 

compliance monitoring within 30 days.

What are the Compliance Monitoring Requirements for Chemical 

Disinfection?

GWSs Serving 3,300 or Fewer

► GWSs using chemical disinfection and serving 3,300 or fewer persons must monitor for the 

residual disinfectant concentration and meet the State specified minimum concentration 

at or before the first customer.

► GWSs must monitor on a daily basis and collect a grab sample during the hour of peak flow 

or at another time specified by the State.

 If any daily grab sample is less than the minimum disinfectant residual concentration, 

the system must take follow-up samples every four hours until the residual meets or 

exceeds the State-specified minimum concentration.

 These systems also have the option to monitor continuously.

• If the GWS monitors continuously, the system must meet the monitoring 

requirements for GWSs serving greater than 3,300 persons (see below).

► GWSs must monitor at a State-approved location.
Office of Water (4606) EPA 815-F-08-008 www.epa.gov/safewater July 2008

GWSs Serving Greater than 3,300 Persons

► GWSs using chemical disinfection and serving greater than 3,300 persons that conduct compliance monitoring must monitor 

for the residual disinfectant concentration and meet the State specified minimum concentration at or before the first 

customer.

 GWSs of this size must monitor continuously and record the lowest residual disinfectant concentration each day that 

water from the ground water source is served to the public.

► GWSs must monitor at a State-approved location.

Failure of Continuous Monitoring Equipment

► In the event of equipment failure for continuous monitoring, provisions are available for all GWSs serving greater than 3,300 

persons and GWSs serving 3,300 persons or fewer who opt to monitor continuously. 

 If there is a failure in continuous monitoring equipment, the ground water system must conduct grab sampling every 

four hours until the continuous monitoring equipment is returned to service. 

 The system must resume continuous residual disinfectant monitoring within 14 days.

What are the Compliance Monitoring Requirements for Membrane Filtration?

► GWSs that use membrane filtration systems to achieve 4-log virus treatment to meet GWR requirements must:

 Operate the process in accordance with State-specified compliance requirements.

 Monitor the membrane filtration process in accordance with all State-specified monitoring requirements.

 Verify that the integrity of the membrane is intact.

► The frequency and location of samples for systems conducting membrane filtration will be determined by the State.

What are the Compliance Monitoring Requirements for Alternative Treatment?

► GWSs that use alternative treatment systems to achieve 4-log virus treatment to meet GWR requirements must:

 Operate the process in accordance with State-specified compliance requirements.

 Monitor the process in accordance with State-specified monitoring requirements.

Compliance Monitoring and Validation Testing for Ultraviolet (UV) Disinfection

► GWSs using UV disinfection as an alternative technology to meet GWR requirements should:

 Monitor for UV intensity, as measured by a UV sensor, flow rate and UV lamp status and any additional State-specified 

parameters.

 Verify the calibration of UV sensors, and recalibrate in accordance with a State-approved protocol, at least monthly.

► UV reactors should undergo validation testing to determine the operating conditions under which the reactor delivers the UV 

dose corresponding to the virus log removal credit received. See “Ultraviolet Disinfection Guidance for the Final Long Term 

2 Enhanced Surface Water Treatment Rule” (http://www.epa.gov/ogwdw/disinfection/lt2/pdfs/guide_lt2_uvguidance.pdf) for 

more information.

Summary

► The following table summarizes the compliance monitoring requirements for systems providing 4-log virus treatment in 

lieu of triggered source water monitoring or as a corrective action under the GWR.

System Type Monitor For Frequency Sample Location

GWSs serving ≤ 3,300 using 

chemical disinfection Residual disinfectant concentration 

(must meet State minimum)

Daily or continuous State-approved 

GWSs serving > 3,300 using location(s)

chemical disinfection Continuous only

GWSs using membrane filtration Membrane filtration process 

effectiveness Consult State for specific information GWSs using State-approved 

alternative treatment Alternative treatment effectiveness

► If operation according to the criteria or requirements for compliance monitoring (minimum residual disinfectant concentration, 

membrane operating criteria or membrane integrity, alternative treatment operating criteria, etc.) is not restored within four 

hours, a GWS must notify the State as soon as possible.

► For all GWSs conducting compliance monitoring, failure to conduct required compliance monitoring (Sec. 141.403(b)) 

requires a Tier 3 public notice.

► If any GWS wishes to discontinue 4-log treatment of viruses before or at the first customer, the GWS then becomes subject 

to the GWR triggered source water monitoring requirements. 

 See “Ground Water Rule Factsheet: Monitoring Requirements” and “Ground Water Rule Triggered and Representative 

Monitoring: A Quick Reference Guide” for more information.