Smelly water in underground spring-fed spring box, cistern

We get our water from an underground spring-fed spring box, cistern; it is about 15 feet deep. The water has lately developed a foul odor. We have checked the box for foreign objects (eg: decomposing animals) but nothing can get in because the lid is a 4’x4’x4” reinforced concrete slab. We don’t drink the water but use it for bathing and washing. We are in a severe drought here in North Georgia and have been conserving water, so some neighbors have said it is because we do not use enough water to refresh it. I am thinking maybe we need to pump it dry and wash the cistern. Any ideas and/or suggestions would be appreciated.
Thanks snipped-for-privacy@gmail.com
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snipped-for-privacy@gmail.com wrote:

Get a full range test. It could just be sulfur from the rock.
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