Water in Elevator Shaft

You did not say how far below grade your elevator shaft extends. Excavations in urban areas often encounter ground water high in H2S. I suspect that the ground water entering the sump is elevated in the sulphides and not just the incidental water not being drained to the sump. The rust like deposits are likely the result another nuisance bacteria called iron bacteria.

H2S is caused by the reduction of organic matter under anaerobic conditions by sulphur reducing bacteria (SRB). These bacteria thrive in biofouled environments. Iron bacteria will create a coating of slime on the formation. Underlying the iron bacteria slime is an ideal environment devoid of oxygen where SRB thrive. As the biofilm sloughs H2S enters the water and releases its wonderful aroma. Often iron sulphide will be evident as black deposits and often slimy like a runny tar. Why do we notice it more in an urban environment? One reason may be that some of the sewers are old and leaking into the underlying formation. This organic matter then becomes the food for the nuisance bacteria. SRB are very clever and need only a small amount of organic to grow. The organic need not come from exfiltration of the sewers but could be from low levels of natural TOC or total organic carbon although I suspect the sewers as part or all of the problem. The presence of iron and sulphur together indicates an advanced level of these bacteria and lots of biofouling.

What cure is there? If you can rectify the drainage you may still be faced with a sump that smells. As an aside, I did work on a hospital with a sump about 35 feet below grade. It was a new wing on a very old building. The new sump had very high levels of H2S (>10 mg/L) and resulted in the hospital wing being closed and not officially opened. Copper pipe newly installed had blackened as evidence of the airborne sulphide. This sump was discharged to the storm drain (not sewage line) that collected water from curbs, roofs and other buildings in the area. When the hospital discharged it created smells in neighbouring buildings and led to the close of the library because of H2S release in that building.

The best cure we had was to install a chemical pump that fed 35% peroxide on a timed bases to control the odour. It was very effective. Peroxide has the added mechanical feature of bubbles being released that helps to penetrate the biofilm. It is also environmentally friendly and can be discharged to storm drains. It will degrade rapidly in a storm drain environment.You may want to try hand dosing (say 1 liter of 35 % mixed in a 20 liter pail of water) the sump before installing anything more permanent. However I do suspect you will need more than the odd swabbing of the elevator shaft to achieve good results long term. Good Luck. Here are the reactions.

8 H₂S(g) + 8 H₂O₂(aq) = S₈(s) + 16 H₂O(l)

or in more alkaline environments

S^2-(aq) + 4 H₂O₂ (aq) = SO₄^2- (aq) + 4 H₂O(l)

Peroxide is best do not use a hypochlorite or you will not be able to discharge it to storm drains.