Standpipe Installation after borehole drilling

[qhtony]

Hi guys... I've encountered a situation that I finished drilling a borehole to 15m and the driller started backfilling the borehole with dirts/soil coming out of the hole and some sand. As soon as he finished backfilled the hole, I realised that I would still need to install a standpipe to do some GW chemical testing (OMG, I did forget). That measn I will have to drill another hole to put in my standpipe (no SPTs or tubes required, of course).

So here is the question, if I drill the hole right at my old borehole (the one backfilled) location and install the the standpipe, will it affect the GW chemical testing results much ??

My thought is that it'll take less time to drill for the purpose of installing a standpipe because the backfilled materials are loose and it won't contaminate the GW because the materials we backfilled with were from the bore itself and some clean sand. And I can't think of anything that can potentially contaminate the borehole. Also, before we take GW from the standpipe for testing, we purge 3 Volumes of water as well.

So I'm just wondering if it's acceptable to drill at the old borehole location or it's necessary to drill a borehole right next to the old bore (which will take longer time)??

Thanks

 

[fattdad]

It's necessary to drill a new hole. You'll pay either way and the fresh cut will be better for your monitoring well development and ultimate sampling. Cost difference, if any, will be insignificant compared to any unknown perceptions.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

fattdad's point is well taken - and it addresses any future arguments that could arise if the water test results were "out" and you had used the "contaminated" borehole. Best in situations like this to be a virgin!

 

[qhtony]

Thanks guys...I'll just drill the new hole next to the old one then...

 

[dgillette]

There's some risk of 'contaminating' the bottom of the hole where it's screened if spoils from near the surface get in; the cuttings probably got mixed together. This would create potential for red herrings from shallow lead from pre-1974 cars, blowing industrial dust or fertilizer, etc.

Regards,
DRG

 

[DirtMike]

Drilling a new borehole probably won't take much if any additional time anyway if you are drilling in soil for the full depth, with the exception of highly overconsolidated clays. Any time savings wouldn't outweigh the risk of cross-layer contamination from mixing cuttings from different depths during the initial backfilling.

 

[looksatstars]

if your going to drill within 10m of the previous borehole, I would also benonite seal the previous borehole at the top to prevent surface water from contaiminating the ground water underneth. This of course is depending what you are checking for.
a bag of bentonite is cheap compared to erroneous results.