"Plant uptake of arsenic in vegetables grown in garden soils contaminated with arsenic from mining activity in south-west England has also been reported (Thornton, 1994). In this study, 32 home garden sites were examined. The soil arsenic at these sites ranged from 144-892 mg/kg and averaged 322. In normal agricultural soils in England the range is 2-53 mg/kg As with an average of 10 mg/kg As. The arsenic content was determined in six garden crops: lettuce, onion, beetroot, carrot, pea and bean. The arsenic uptake (mg/kg dry weight) was species dependant with the highest amount in lettuce (average 0.85, range 0.15-3.9, n=28) and lowest in beans (average 0.04, range 0.02-0.09, n=7). The effects of soil constituents such as iron, phosphorus, and calcium on arsenic uptake were examined. In lettuce, the uptake increased with increasing phosphorus in the soil and decreased with increasing iron content, presumably due to competitive sorption reactions between phosphorus and arsenic in the soil and with precipitation reactions with the iron to form insoluble iron arsenates. The UK statutory limit of 1 mg/kg As (fresh weight) was not exceeded in any of the vegetable samples."
taken from: http://www.noccawood.ca/stilwell1.htm
So... it would seem that Iron is your Friend, which is good because a bag of iron sulphate is readily available at almost all gardening stores and is really inexpensive (it`s usually sold as Moss killer).