The iPod-carrying lifeguard watching over the rooftop pool at the posh Park Connecticut apartments, the physics teacher at Suitland High School and several laborers all have one thing in common: They are among the foreign workers who keep the local economy running, turning up more and more often in unexpected jobs. Arlington's High Sierra Pools, which once counted on eager American teenagers to occupy its lifeguard chairs, now fills nearly half of its 500 jobs with young people from Bulgaria, Poland, Peru and elsewhere.