Groundwater Under Direct Influence (GWUDI)

What is GWUDI and what systems are impacted?

GWUDI classified spring located in a marsh that tested positive for E. coli.

One of the most costly decisions we can make for a water system is whether or not to classify a water source as ground water under the direct influence of surface water (GWUDI). However, making this determination correctly is critical to protecting public health. Back in 2012, when many Colorado wells had not yet been evaluated for GWUDI, the state’s drinking water program developed Policy 3, which set the process for evaluating water sources for GWUDI. Policy 3 was designed to ensure consistent determinations that would protect public health without overburdening water systems.

What is GWUDI?

The program's Policy 3 defines GWUDI as any water beneath the surface of the ground with:

• Significant occurrence of insects or other macroorganisms, algae, or large-diameter pathogens such as Giardia lamblia or Cryptosporidium; or
• Significant and relatively rapid shifts in water quality parameters such as turbidity, temperature, or conductivity which closely correlate with adjacent surface water or climatological conditions; or
• Any source that was unable to pass a visual well inspection.

How Do We Evaluate Systems?

Typically, we refer systems for further evaluation based on sanitary surveys, engineering plans reviews, discovered systems or sources, customer inquiries, and well samples that test positive for E. coli. After our initial screening, systems typically fall into one of the following 3 categories.

1 - Remain Classified as Groundwater 

For this scenario, systems typically need to meet all of the following conditions:

• The source adequately passes a visual well inspection 
• The source depth is greater than 50 feet 
• The groundwater flowpath is greater than 500 ft to surface water
• Aquifer recharge activities are occurring at greater than 500 ft from the source
• The time of travel is greater than 50 days

2 - Immediately Reclassified as GWUDI  

In this scenario, water quality data (e.g., presence of E. coli data, diatoms, algae, etc.) and source infrastructure data (e.g., newly discovered infiltration galleries, construction of the source, proximity to surface water, etc.) clearly indicate surface water intrusion.

3 - Conduct Performance Monitoring 

Under this scenario, the division sets a monitoring schedule for performance data such as temperature, total coliform, and microscopic particulate analysis (Table 1) to be collected. Once the required monitoring is finished, the division evaluates the monitoring data and makes a determination that also takes into consideration well history, completion logs, proximity to surface water, and overall pumping rates. When necessary, the division convenes a team of reviewers to make the determination.

 

Table 1 - Groundwater Performance Monitoring Requirements

Parameter	Location	Frequency	Sampling Dates
Temperature and conductivity	Well and surface water (if available)	2X per 7-day week	April through October
Raw water total coliform and E. coli.	Well	1X month	April through October
Microscopic particulate analysis	Well	3 times as specified	1st in Apr/May, 2nd in June/July, 3rd in Aug/Sept
Aerobic spores	Well and surface water (if available)	3 times as specified	1st in Apr/May, 2nd in June/July, 3rd in Aug/Sept concurrent with MPAs

 

How many systems have been evaluated for GWUDI?

The division has evaluated 543 systems since implementing Policy 3 in 2012. We reclassified 90 of those as GWUDI and required 276 to conduct performance monitoring (Table 2).

 

Table 2 –Evaluation Results from 2012 through 2019

Start Year	Total Evaluated	Performance Monitoring Required	Groundwater	GWUDI
2010-2012	118	9	3	19
2013	70	35	88	10
2014	45	41	60	10
2015	57	32	34	13
2016	59	30	44	12
2017	64	37	38	15
2018	52	22	38	8
2019 (to date)	78	40	32	3
Totals	543	276	35	90

 

Years 2010-2012 contain historical cases

• All years include systems classified as GWUDI without sampling due to failing to collect the required data or voluntarily accepting a GWUDI classification.
• All years include systems that were classified as groundwater without sampling.
• Most years have systems that completed the monitoring over two years.
• All years have cases that were canceled.
• Year 2019 data is not yet complete.

What are the impacts of Policy 3? 

Since 2012, evaluating water sources for GWUDI has protected at least 75,000 people from possible illness caused by ingesting improperly treated surface water. Although most water systems resist performance monitoring at first, these data collection efforts often save systems money. The typical cost of monitoring a single source is $2,000-$3,000, which is far less than the tens of thousands it would cost to install a surface water treatment system after an incorrect GWUDI determination due to lack of data collection. Furthermore, performance monitoring under Policy 3 gives water systems easy-to-interpret, data-driven final reports.

➽ Bryan Pickle, groundwater evaluation specialist