Arsenic, a past common corrosion inhibitor in petroleum fields, was used in two different ways. First, arsenic was the main corrosion inhibitor for HCL acid jobs from 1932 to the early 1960s, and then limited to high-temperature wells into the early 1970s. About 2,000 mg/l of sodium arsenite was used in a typical acid job. Secondly, from 1949 until the early 1970s, arsenic was mixed and often circulated in the produced water of an oilfield system. This latter usage varied and included 1) introducing up to 50-60 mg/l dissolved arsenic at a well for circulation of about 5 mg/l dissolved arsenic within a produced water flow system; or 2) placing dissolved arsenic (1,000 mg/l or more) and brine within temporarily-abandoned wellbores to prevent corrosion. Oilfield arsenic entered the environment from surface spillage, earthen pit disposal, saltwater flowline breaks, and waste burial. Modern technical documents on arsenic at petroleum industrial-impacted sites often fail to list this potential anthropogenic source—American Petroleum Institute publications from 1998 and 2011 are examples. Company documentation of past arsenic usage has been found for four oilfields and one acid-mixing facility. Three of the five sites have modern available cleanup data. All of the oilfields, three in California and one in Louisiana, were historically operated by Standard Oil of California who used the W-41 arsenic corrosion inhibitor. Lake St. John (LSJ) Field, LA, has soil and shallow groundwater geochemical data from 2007-2016 available due to litigation around three old pit areas. The field is located on the Mississippi River alluvium. Total arsenic values in shallow groundwater below and near the pits vary from non-detect up to 0.915 mg/l. Two models propose explanations for shallow groundwater arsenic patterns. A reductive-dissolution model was put forward by environmental experts and submitted to the LA regulatory records from 2008 to 2015. Another model, proposed in 2014 by the author, was based on historic company records which state that W-41 was used here. Both elevated groundwater arsenic and iron are remnant corrosion products rather than related to reductive-dissolution of absorbed natural arsenic on iron. Pit geochemical patterns indicate that relative ion mobility in groundwater is chlorides > iron > arsenic.