Her body has not been found.
Ron and Sharon Whited still refer to their daughter, who was married with two children, as "bright eyes," a nickname she picked up in elementary school. In her absence, the Whiteds said they've been bolstered by an outpouring of support from friends and the solace offered by the pastor at their church, Trinity Lutheran.
Sidney Schools superintendent Dan Farr, who was trained as a school counselor and worked with Arnold for 13 years, said the continued mystery of her whereabouts has provoked a particular form of grieving called "ambiguous loss" that robs family and friends of closure because there is no body to bury.
For the school district, the loss of a beloved teacher is set against a backdrop of skyrocketing enrollment from workers who moved to Sidney with family in tow. Over the next two to three years, Farr said, Sidney's population could double if proposed new subdivisions, RV parks and "man camps" for workers are built. Student numbers are projected to climb more than 60 percent.
Sidney Mayor Bret Smelser is pushing for more oil revenues to be returned to towns and cities hardest hit by the boom. For now, much of that money goes to counties, which Smelser said denies him of resources for his community.
A glimpse of Sidney's future can be seen in the experience of Williston and surrounding Williams County, N.D., where more than 9,000 beds have been permitted for man camps, sprawling compounds of trailers or mobile homes that companies temporarily erect in open fields for worker housing. Williams County Sheriff Scott Busching said that calls to his department have risen sharply during the last three years, forcing him to double patrol deputies from 10 to 20.
That includes spikes in traffic accidents and aggravated assaults linked to bar fights. In response, many local residents are arming themselves against potential danger. Concealed weapon permit applications in Williams County soared from 156 in 2010 to 550 last year, the sheriff said. Arnold's disappearance has further accelerated the trend, with 126 new applications coming in January alone.
Montana authorities are seeing similar trends emerge. Sidney Police Chief Frank DiFonzo said the added stress on his force has made his officers more reactive than proactive, with little time for once-routine criminal investigations.
DiFonzo, Sidney's chief since 1981, said the increased workload appears to reflect the sheer number of new arrivals, rather than an increase in particular crimes. And though the oil industry is what's bringing those workers, DiFonzo said it would be no different if they were seeking sugar beets or gold.
"I'm going to give them the benefit of the doubt that they're coming here to work," he said. "But it's made the residents who live here very nervous."
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