Turbidity MCL for Groundwater NOT UDI of surface water 1

[dnsalyer]

This is a drinking water question:
The regulations are clear regarding the requirements for controlling turbidity of surface water or ground water under the direct influence (GWUDI) of surface water that is being treated by filtration processes. What is unclear is if these same turbidity requirements apply to groundwater treatment systems where the groundwater is NOT under the direct influence of surface water and is not blended with surface water or GWUDI after treatment.

What is the turbidity MCL in this case?
Is it even required by the EPA to monitor turbidity in such cases?

Thanks for any help,
Dave Salyer

 

[bimr]

The USEPA Primary Drinking Water Standard for turbidity is a maximum contaminant level of 0.5 NTU (not to exceed 5% of monthly samples). This applies to all drinking waters including ground water sources.

Turbidity monitoring requirements are much less extensive for ground water applications than for surface water applications, since ground water by definition should have been filtered naturally. There is no need to waste money monitoring when the ground water quality is good.

However, when your turbidity monitoring for ground water indicates a problem with your water supply, then the reviewing agency converts your ground water source into the GWUDI classification. The reviewing agency typically specifies the requirements for this reclassification. And the water monitoring requirements are cranked up.

You need to check with the reviewing agency where the project is located for the specific requirements such a frequency of monitoring.

 

[dnsalyer]

Thank you Bimr. I believe the MCL for turbidity for systems serving >10,000 people is actually 0.3 NTU for 95% of the samples, with a not to exceed of 1 NTU. For <10,000 people, the 95% is 0.5 and the not to exceed is 5 NTU.

The problem I have with the regulation is that there is no specific monitoring rule that I'm aware of. 95% of how many samples?

Keeping under 0.3 or 0.5 is simple enough for a pressure filter, but spikes can occur around backwash that can exceed 1 NTU momentarily. That can make operating the system a little bit of a challenge.

I already have requests in with the local agency and the USEPA for clarification.

Thanks again for another of your usual, informative posts.

 

[bimr]

Isn't that the MCL for the surface water rule?

The drinking water requirements are somewhat confusing as shown in the example from a typical state:

http://www.health.state.ny.us/environmental/water/drinking/part5/tables.htm#table1

However, I think that the answer to your question is a sample frequency of just one per month. That would be the minimum sampling requirement. That answer is by default since your application does not fall into the surface water rule or GWUI rule.

The best answer is going to come from the reviewing agency where the project is located. If your system is already operating, the requirements should be listed on the operating permit.

As to the turbidity spike on backwash, have you investigated the use of a filter-to-waste step? I just went through that issue with a gravity system.

 

[dnsalyer]

Thanks again Bimr. You are right, that is the MCL for surface water. I have not specifically found any other MCL and until now I could only assume that it also applied to groundwater. The example you sent for NY state seems clear to me in that turbidity monitoring is not applicable to filtered water when the source is groundwater not UDI (Table 10A).

Our state has gotten back with me and confirmed they do not consider the regulations for turbidity applicable to groundwater that is not UDI and that there are no requirements for turbidity monitoring.

Nevertheless, turbidity control is still an important issue and customers need to have clear water. My feeling is that turbidity spikes of 5 NTU should be tolerable if they are short-lived. Distribution systems may have turbidity limits separate from specific treatment plants or POEs and for that reason alone, filter plant operators should keep a close eye on turbidity. Also noteworthy is the fact that turbidity can indicate problems with a filter that can be caught before significant damage is done.

Running to waste prior to returning to service is a proven way of avoiding turbidity spikes. Unfortunately that is not an option in this particular case.

We have managed to minimize the spikes by keeping a close watch on valve transitions and other timed events and can keep the spikes at or below 1 NTU throughout the backwash. My original query was related to occasional excursions above 1 NTU and whether they would put us out of compliance from a turbidity standpoint; I believe now that this is not a compliance issue per se.

Thanks again,
Dave Salyer