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We present composite spectra constructed from a sample of 242,150 Lyman-alpha(Lya) forest absorbers at redshifts 2.4<z<3.1 identified in quasar spectra fromthe Baryon Oscillation Spectroscopic Survey (BOSS) as part of Data Release 9 ofthe Sloan Digital Sky Survey III. We select forest absorbers by their flux inbins 138 km/s wide (approximately the size of the BOSS resolution element). Wesplit these absorbers into five samples spanning the range of flux -0.05 <F<0.45. Tests on a smaller sample of high-resolution spectra show that ourthree strongest absorption bins would probe circumgalactic regions (projectedseparation < 300 proper kpc and |Delta v| < 300km/s) in about 60% of cases forvery high signal-to-noise ratio. Within this subset, weakening Lya absorptionis associated with decreasing purity of circumgalactic selection once BOSSnoise is included. Our weaker two Lya absorption samples are dominated by theintergalactic medium.  We present composite spectra of these samples and a catalogue of measuredabsorption features from HI and 13 metal ionization species, all of which wemake available to the community. We compare measurements of seven Lyman seriestransitions in our composite spectra to single line models and obtain furtherconstraints from their associated excess Lyman limit opacity. This analysisprovides results consistent with column densities over the range 14.4 <~ Log(N_HI) <~ 16.45. We compare our measurements of metal absorption to a varietyof simple single-line, single-phase models for a preliminary interpretation.Our results imply clumping on scales down to ~30 pc and near-solarmetallicities in the circumgalactic samples, while high-ionization metalabsorption consistent with typical IGM densities and metallicities is visiblein all samples.

Probing the Circumgalactic Medium at High-Redshift Using Composite BOSS Spectra of Strong Lyman-alpha Forest Absorbers