DES MOINES, Iowa (AP) - Rivers that supply Des Moines' drinking water are still showing high nitrate levels, prompting officials to turn to a reservoir and other sources for water.Des Moines Water Works has already spent $500,000 removing nitrate this year and said the persistent problem likely will lead to higher water rates for customers to pay for the additional cost of treatment, The Des Moines Register reported (http://dmreg.co/14Is9fE ).The city typically draws water from the Raccoon and Des Moines rivers and mixes it with water from a reservoir to keep nitrate levels lower.Most of the nitrate gets into the water when rain flushes fertilizer and animal manure out of farm fields. The problem is worse this year because it has been an unusually wet spring following a drought during which stunted corn plants didn't absorb all the nitrogen-rich fertilizer placed on fields.Under Environmental Protection Agency requirements, municipal water providers must keep the level of nitrate in drinking water below 10 milligrams per liter, as anything above can be deadly to infants younger than 6 months because the chemical can reduce the amount of oxygen carried in their blood.To keep the water delivered to area homes below the EPA standard, the water works has stopped drawing river water and is pulling water from Maffitt Reservoir, a 200-acre lake.During the first week of May, the Raccoon River posted a nitrate level of a record 24 milligrams per liter and the Des Moines River reached a record of 18 milligrams per liter. Those levels dropped when measured last week - 14.4 milligrams per liter for the Raccoon and 16.4 for the Des Moines. Both rivers have been above the EPA standard for about two months.Nearly always, when one river has high nitrate levels, the other is lower, giving the water utility some options, including blending water from backup supplies with the cleaner river water.But this year, Stowe is running out of options."The issue is we have never seen these levels," he said.To reduce the nitrate level, Stowe in early May turned on a $4 million nitrate removal system built in 1991. The system costs $7,000 a day to run and has so far been able to barely keep up.Neighboring West Des Moines Water Works has bumped up water production by about 20 percent from its wells to take pressure off the Des Moines system.Nitrate surges during spring rain are not uncommon but this year's prolonged spike appears unusual."I suspect that the repeated heavy rainfall events and cold temperatures late in the spring, which have suppressed corn growth and nitrogen uptake, have created a situation where the nitrogen continues to be leached out of the system because the corn crop is substantially behind this year," said Mary Skopec, a member of the Iowa Department of Natural Resources water monitoring staff.